Endeavoring the First Lanthanide–Carbon Triple-Bond in Fullerene Cage

Endeavoring the First Lanthanide–Carbon Triple-Bond in Fullerene Cage | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Endeavoring the First Lanthanide–Carbon Triple-Bond in Fullerene Cage Jun Li, Ning Chen, Hongjie Jiang, Jing Zhao, Qingyu Meng, Xiao-Kun Zhao, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5351349/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Jun, 2025 Read the published version in Nature Chemistry → Version 1 posted You are reading this latest preprint version Abstract Metal–ligand multiple bonds are intriguing in coordination and organometallic chemistry. However, lanthanide–carbon multiple bonds are extremely difficult to form. Despite decades of effort, isolable complexes containing lanthanide–carbon double bonds with terminal methyl carbene (= CH 2 ) and lanthanide–carbon triple bonds have never been found. Here, we report the successful synthesis of an unprecedented lanthanide–carbon triple-bonded compound with cerium-carbide [Ce≡C − Sc 2 ] cluster encapsulated inside a C 80 fullerene cage. The molecular structure of CeCSc 2 @C 80 and the nature of the Ce≡C triple bond are characterized through X-ray crystallography, spectroscopic analyses, and quantum chemical study, revealing a very short Ce≡C distance of 1.978(8) Å. Chemical bonding analysis suggests that the formation of the Ce≡C bond primarily arises from the stronger bonding affinity between carbon and cerium compared to scandium inside the encapsulated cluster. The fullerene cage plays a crucial role in stabilizing and protecting this trimetallic carbide cluster with a Ce≡C triple bond. Physical sciences/Chemistry/Inorganic chemistry/Chemical bonding Physical sciences/Chemistry/Coordination chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Chemical bonding is at the heart of chemistry 1 . Substantial progress has been achieved in understanding the chemical bonds of main-group and transition-metal elements over the decades, particularly through the synthesis of molecular compounds with metal–ligand multiple bonds 2 . However, lanthanides (Ln = La–Lu) of the 4f series, which have significant industrial roles in catalysis, magnets, photonics, alloys, and energy 3 – 5 , are viewed as less likely to form multiple bonds with ligands due to their rather contracted 4f orbitals and limited radial extension of valence orbitals 6 , 7 . Lanthanides exhibit fairly uniform behavior throughout the series, mainly forming trivalent cations that are characterized by ionic and highly polarized bonding 8 . Moreover, the energy levels of lanthanide 5d valence orbitals are notably higher than those of transition metals, making lanthanide metal–ligand multiple bonds highly reactive and extremely difficult to be stabilized due to their low covalency and strong charge polarization 9 . Despite these challenges, significant efforts are made to the synthesis of complexes with lanthanide–ligand multiple bonds. Over the past two decades, several successful syntheses of alkylidenes 10 – 15 , alkylidynes 16 , 17 , imido 18 – 23 , and oxo 24 – 27 compounds have contributed to the understanding of lanthanide–ligand multiple bonding. However, due to their high Lewis acidity, oxophilicity, and large coordination spheres, the multiple bonds formed between lanthanides and carbon are generally highly polarized and possess low covalency and bond order 8 . Cerium, which can be stabilized in a tetravalent oxidation state (Ce IV ), has been considered a promising candidate among lanthanides for forming lanthanide–ligand multiple bonding 8 , 11 , 17 – 19 , 21 . However, theoretical and experimental investigations regarding Ce = C double bonds and the role of 4f orbitals in bonding remain controversial. Previous theoretical calculations suggested a Ce = C bond length of 2.127 Å in Cp 2 CeCH 2 28 , which was assumed to be in a closed-shell singlet state. The Ce = C double bond distance of 2.042 Å was predicted for CeCH 2 + cation 29 . However, more comprehensive theoretical studies have indicated that Ce and methylene do not form terminal double bonds but rather generate a “substituted methyl radical” H 2 C•−•CeL 2 (L = F 30,31 , Cp 32 ), exhibiting characteristics of a non-Lewis paired diradical species as demonstrated in U–Si complexes with so-called multi-radical bonding (MRB) 33 . This kind of open-shell singlet character of H 2 C•−•CeF 2 complex 30 might provide a hint at the repeated unsuccess in synthesizing compounds with terminal Ce = C double bonds. To date, only two compounds featuring predominantly electrostatic Ce = C double bonds have been reported, with bond lengths of 2.441 Å in [Ce(BIPM TMS )(ODipp) 2 ] 12 and 2.385 Å in [Ce(BIPM TMS ) 2 ] 15 . These bond distances are only slightly shorter than the s-single-bond distances in the recently synthesized Ce(IV)-alkyl, -aryl, and -alkynyl complexes, where the Ce IV –C σ-bond distances lie between 2.48–2.54 Å 34 . While lanthanide metal–carbon terminal multiple bonds with higher covalency had yet to be achieved, Ln≡C triple bonds, which seem to be beyond the bonding ability of 4f elements, appear unlikely to be formed. The effort towards preparing compounds with actinide triple bonds had been unsuccessful. However, our studies suggest that fullerenes can act as nano containers and effectively stabilize bonding motifs which otherwise are not available in conventional compounds. We have designed the strategy by making use of fullerene confinement to isolate U≡C bonds 35 – 37 . These results further inspired us to explore the possibility of stabilizing Ln–C multiple bonding, which can provide insights into the fundamental bonding nature of lanthanides. Herein, we report the synthesis and characterization of the endohedral metallofullerene (EMF) compound, CeCSc 2 @C 80 , which contains a unique Ce≡C triple bond. Through X-ray crystallographic analysis, the structural characterization unveils a Ce≡C bond of 1.978(8) Å, which is the shortest ever recorded to date. The nature of the Ce≡C triple bond has been investigated by spectroscopic methods and quantum-theoretical analyses. Results and Discussion Synthesis and Crystallographic Characterization of CeCSc 2 @C 80 CeCSc 2 @C 80 was synthesized by a modified Krätschmer-Huffman direct-current arc-discharge method 38 . The graphite rods, packed with CeO 2 , Sc 2 O 3 and graphite powder (molar ratio of Ce:Sc:C = 1:1:24), were vaporized in an arcing reactor under a 110 A direct current and 200 Torr He atmosphere. The targeted Ce-based metallofullerene was extracted from the carbon soot by CS 2 solvent for 12 hours and then isolated by multi-step high-performance liquid chromatography (HPLC) (Supplementary Fig. 1). The high purity of CeCSc 2 @C 80 was confirmed by chromatograms and mass spectra (Supplementary Fig. 2). The experimental isotopic distributions of CeCSc 2 @C 80 agree well with the theoretical simulation, confirming its elemental composition. Black block cocrystals of CeCSc 2 @C 80 via strong π-π interactions with Ni II (OEP) (OEP = 2, 3, 7, 8, 12, 13, 17, 18-octa-ethyl-porphyrin dianion) were obtained by the slow diffusion of a benzene solution of [Ni II (OEP)] into a CS 2 solution of CeCSc 2 @C 80 . The molecular structure was solved in the monoclinic space group C 2/ m and determined to be CeCSc 2 @ I h (7)-C 80 by single-crystal X-ray diffraction analysis (Fig. 1 A and Supplementary Table 1). CeCSc 2 @ I h (7)-C 80 has two orientations of carbon cages with the same occupancy of 0.5, due to the crystallographic mirror plane in the C 2 /m space group. As to the inner cluster, the central carbide atom, C81, is situated on the mirror plane and fully ordered with an occupancy of 0.5. There are three crystallographically disordered sites for Ce atoms (Ce1, Ce2, and Ce3) and four sites for Sc atoms (Sc1, Sc2, Sc3, and Sc4). Additionally, due to the presence of crystallographic mirror planes, mirror-symmetrical atoms are generated correspondingly (Supplementary Table 2). Among these, the major metal sites for Ce atoms are at Ce1, with an occupancy of 0.4035(15), while for Sc atoms, they are at Sc1 and Sc2, with corresponding occupancies of 0.435(4) and 0.324(7), respectively. Hence, the inner cluster exhibits relatively high occupancy, ensuring the accuracy of subsequent analysis of the cluster structure and chemical bonding in CeCSc 2 @ I h (7)-C 80 . In CeCSc 2 @ I h (7)-C 80 , the distances between two Sc atoms and the nearest carbon atoms on the cage are 2.166 Å and 2.205 Å, respectively. The distances between Ce and the carbon atoms in the adjacent hexagonal ring range from 2.478 Å to 2.619 Å, which are within the Ce–C single bond distance. Overall, the CeCSc 2 cluster nearly forms a planar triangle with a total bond angle of 359.6° (Fig. 1 B), and the distances between C81 and Sc atoms in the cluster are 2.010(7) Å and 2.035(9) Å, respectively. Furthermore, the interatomic distances and angles, including the bond lengths of C cage −Ce, Sc − C cage , C81 − Sc, and the Sc1 − C81 − Sc2 − Ce dihedral angles in the theoretically optimized spin-singlet CeCSc 2 @ I h (7)-C 80 molecules ( vide infra ), all fit well with the experimental values (Supplementary Tables 3–4). The most striking feature of the molecular structure of this fullerene compound is the very short Ce–C bond distance measured as 1.978(8) Å, the shortest cerium–carbon bond distance ever reported in known compounds. This bond distance is shorter not only than the theoretical distance of 2.4 Å for a Ce − C single bond 39 , suggesting its multiple bonding character, but also shorter than the theoretical distances of 2.127 Å for C = Ce in Cp 2 Ce(CH 2 ) 28 and 2.042 Å for Ce = C terminal double bonds in CeCH 2 + cation 29 , as well as the sum of Ce and C double-bond covalent radii of 2.04 Å 40 , indicating an even higher bond order. Furthermore, compared to the experimentally reported Ce = C bond distances of 2.441 Å in [Ce(BIPM TMS )(ODipp) 2 ] 12 and 2.385 Å in [Ce(BIPM TMS ) 2 ] 15 , the remarkably short cerium–carbon bond length of 1.978(8) Å in this compound appears to imply a significantly stronger interaction between Ce and C atoms. Additionally, the experimentally determined distances agree well with the Ce − C81 bond lengths computationally optimized for CeCSc 2 @C 80 (Table 1 and Supplementary Table 4). Altogether, these results provide evidences for the presence of a cerium–carbon bond with high bond order in a stable molecular compound. Spectroscopic Properties Fourier Transform Infrared (FTIR) absorption spectroscopy and Visible-Near-Infrared (Vis-NIR) absorption spectroscopy were utilized to support the structural parameters of CeCSc 2 @ I h (7)-C 80 and investigate the bonding nature of the encapsulated CeCSc 2 unit. As shown in Fig. 2 A, the experimental FTIR absorption spectrum of CeCSc 2 @ I h (7)-C 80 agrees well with the corresponding calculated spectrum. In the high wavenumber range of 1600 ~ 800 cm − 1 , dominant stretching vibrations of the I h (7)-C 80 carbon cage are observed. Characteristic vibrational peaks at 1520, 1383, and 1201 cm − 1 in the experimental spectrum correspond well with those theoretically simulated at 1529, 1395, and 1197 cm − 1 , respectively. These features are consistent with the characteristic vibration peaks attributed to the carbon cage vibrations reported in previous studies 35 . In addition, the endohedral CeCSc 2 unit contributes 12 modes, 10 of which are very weak due to the heavy metal. These modes include three translational rocking modes, three rotational modes and three Sc1–C–Sc2 vibration modes (in-plane scissoring, in-plane rocking and out-of-plane wagging) are located between approximately 20–210 cm − 1 , along with one Sc–C–Sc symmetric stretching mod at 697 cm − 1 . The remaining two more intense peaks at 735 cm − 1 and 696 cm − 1 in the experimental spectrum correspond to Ce–C stretching modes and Sc–C–Sc asymmetric stretching, which align well with the theoretically simulated vibrations at 730 cm − 1 and 698 cm − 1 . The Vis-NIR absorption spectra of endohedral fullerenes are primarily dominated by π→π* transitions within the large π system formed by the carbon atoms on the fullerene cage. Hence, the cage isomerism, the charge transfer, and interactions between the fullerene cage and the encapsulated species significantly affect the Vis-NIR absorption spectra of EMFs. As shown in Fig. 2 B, the absorption spectrum of CeCSc 2 @ I h (7)-C 80 appears relatively smooth without significant characteristic absorption frequency. This absorption spectrum is very similar to the absorption of previously reported EMF with I h (7)-C 80 symmetry carbon cages and six cluster-cage six electron transfer 35 . Accordingly, it appears reasonable to assume that the electronic configuration of CeCSc 2 @ I h (7)-C 80 is formally [CeCSc 2 ] 6+ @[C 80 ] 6− . Electron Structure and Bonding Analysis Quantum chemical studies using density functional theory (DFT) were conducted to validate the experimental findings and understand the electronic structures of CeCSc 2 @C 80 . The geometric optimizations with PBE 41 , B3LYP 42 and PBE0 43 functional all reveal that the closed-shell singlet state, 1 A CeCSc 2 @C 80 , is the most stable configuration, with an equivalent chiral isomer 1 A* (Supplementary Table 5 and Supplementary Fig. 3). This singlet state is 13–27 kcal/mol lower in energy than the second-most stable triplet state with different DFT functional. The calculated geometric parameters show fair agreement with experimental data, with the calculated Ce–C bond length of 2.049 Å close to the experimental value of 1.978 Å (Table 1 ) at the B3LYP level. With inclusion of dispersion description of B3LYP-D3BJ, the Ce–C bond length reduces to 2.036 Å. Theoretical bonding analyses reveal that a triple bond forms between Ce and the central C atom in the encapsulated [CeCSc 2 ] unit. The Kohn-Sham energy level diagram of the frontier canonical molecular orbitals (MOs) and corresponding s/p MOs of CeCSc 2 @C 80 , as depicted in Fig. 3 , show that the Ce ≡ C triple bond consists of one σ bond (MO 175) and two π bonds (MO 172–173). The details are given in Supplementary Table 6 and Supplementary Fig. 4. This Ce ≡ C triple bond is primarily contributed by C 2p orbitals, with minor contributions from Ce 4f/5d orbitals and nominal contributions from Sc 3d orbitals. The Natural Localized Molecular Orbital (NLMO) analysis 44 , the principal interacting orbital (PIO) analysis (Supplementary Fig. 5) 45 , and the Adaptive Natural Density Partitioning (AdNDP) analysis(Supplementary Fig. 6) 46 at the B3LYP level all confirm the Ce ≡ C triple bonds. From NLMO analysis shown in Fig. 4 , the Ce ≡ C triple bond within CeCSc 2 @C 80 has Ce atom contributing 28% to the σ bond, and 19% and 17% to the two π bonds. The detailed atomic orbitals (AOs) contributions are provided in Table 2 . These contributions are significantly higher than the 6–14% reported for Ce = C double dative bond in [Ce(BIPM TMS )(ODipp) 2 ] and [Ce(BIPM TMS ) 2 ] 47 , indicating enhanced covalency in the triple bond. Moreover, the calculated Nalewajske-Mrozek (N-M) bond order 48 of 1.99 for Ce ≡ C is substantially higher than those (1.10–1.16) for the reported Ce = C double bonds 47 . Usually, the bond orders (Supplementary Table 7) are smaller than the formal bond order due to significant bond polarity. Especially noteworthy is the results of Natural Resonance Theory (NRT) analysis 49 , which shows that the predominant resonance structure is the Ce ≡ C triple-bond component with a contribution of 60%, and the Ce = C double-bond component accounts for 30% (Supplementary Fig. 7). The quantum theory of atoms-in-molecules (QTAIM) analysis 50 provides a topological perspective to demonstrate the covalent character of the triple Ce ≡ C bond in CeCSc 2 @C 80 compared to other reported compounds with Ce–C bonds (Supplementary Table 8) 12 , 15 , 51 . The electron density ( ρ ), Laplacian of electron density (∇² ρ ), electron energy density ( H ), and delocalization index (DI) at the Ce ≡ C bond critical point (BCP) indicate stronger interactions and increased covalency between Ce and C atoms. The largest ρ of 0.16 and the most negative H of − 0.09 at the Ce ≡ C BCP highlight strong covalent interaction. However, the positive ∇² ρ of 0.16 combined with an η index of 0.44 indicates the polarity of this bond, consistent with the atomic contributions in the canonical or localized orbitals analysis. Additionally, both single and triple bonds typically exhibit cylindrical electron densities around the nuclear bonding axis, resulting in the BCP ellipticity ( ε ) value near zero. The ε for the Ce ≡ C bond in CeCSc 2 @C 80 is 0.12, corresponding to the value of a triple bond, and is significantly smaller than the ε value range of 0.32–0.41 for Ce = C bonds. We have further evaluated the confinement effect of the [CeCSc 2 ] cluster by the fullerene cage. It is found that the fullerene cage plays a crucial role in charge transfer, coordination interactions, electrostatic confinement, and spatial protection during the formation of the trimetallic carbon cluster with a Ce ≡ C triple bond. It turns out that the [CeCSc 2 ] cluster donates six electrons to C 80 , leading to a valence state assignment of [CeCSc 2 ] 6+ @[C 80 ] 6– , because the fourfold degenerate HOMOs (MO 160–163) of I h -C 80 only have two electrons, thus requiring six more electrons to become stable close-shell (Fig. 3 ). 52 Furthermore, the central C atom in [CeCSc 2 ] 6+ accepts four electrons from the connected metal atoms, resulting in formal oxidation states of Sc + III and Ce + IV . To stabilize these high oxidation states, both covalent and ionic interactions between the [CeCSc 2 ] unit and the fullerene cage are essential. The central part of [C 80 ] 6 – has a strong electrostatic potential (Supplementary Fig. 8), providing effective electrostatic confinement for the [CeCSc 2 ] 6+ cation. The Ce 5d orbitals and the Sc 3d orbitals show significant coordination interactions with fullerenes, as demonstrated by PIO analysis (Supplementary Fig. 9). Energy decomposition analysis (EDA) 53 indicates that the interaction between [CeCSC 2 ] 6+ and [C 80 ] 6 – fragments is predominantly electrostatic (68%), with orbital interactions contributing 32% (Supplementary Table 9). Additionally, the fullerene cage provides a matrix-like environment that spatially protects the [CeCSc 2 ] species from redox lability. Replacing the rigid fullerene cage [C 80 ] 6 – with flexible carbon rings capable of accepting electrons does not significantly elongate the Ce–C bond (Supplementary Table 10), indicating that the very short Ce ≡ C bond distance is an intrinsic characteristic of the [CeCSc 2 ] 6+ motif in the appropriate fullerene size. The formation of the counter-intuitive C ≡ Ce triple bond, along with the Sc–C–Sc three-center, two-electron bond, primarily arises from the stronger bonding affinity between carbon and cerium compared to scandium in both electrostatic and orbital interactions. Ce + IV is more positively charged than Sc + III (Supplementary Table 11), resulting in a stronger ionic interaction with C –IV . Moreover, the overlap between Sc 3d AOs and C 2s2p AOs is less effective than that of Ce 5d AOs, leading to a weaker covalent interaction (Supplementary Fig. 10 and Supplementary Table 12). The degree of involvement of Ce 4f orbitals in rare-earth covalent bonding has been long debated. Inasmuch as 4f orbitals are extremely contracted in spatial extension, 7 they can hardly contribute to covalent bonding in rare earth compounds. Indeed, when optimizing the structure of CeCSc 2 @C 80 using a 4f-in-core pseudopotential 54 and corresponding basis set, the structure remains largely unchanged, implying that the role of 4f orbitals is not significant. As Ce 4f and C2s/2p orbitals can hardly overlap at the experimental Ce ≡ C distance of 1.98 Å (Supplementary Fig. 10), the involvement of Ce 4f orbitals in Ce ≡ C bonds is much less than Ce 5d orbitals (Supplementary Table 12). At this very short Ce ≡ C distance, the Ce 4f orbitals start to play a marginal role, albeit non-negligible, in the short multiple bonds between carbon and cerium in this EMF system. Conclusions In summary, the synthesis, isolation and characterization of CeCSc 2 @C 80 are reported herein. This endohedral metallofullerene is shown to feature hereto the first Ce ≡ C triple bond. Solid-state characterization by X-ray crystallographic analysis reveals an exceptionally short cerium-carbon bond of 1.978(8) Å, which is notably shorter than the distances of experimentally reported Ce–C single bonds and Ce = C double bonds distances and is close to the sum of Pyykkö triple-bond covalent radii of 1.91 Å. The computational geometric parameters agree well with the experimental data, confirming the formation of a Ce ≡ C triple bond. The quantum chemical investigations on bonding nature show that the Ce ≡ C triple bond consists of one σ bond and two π bonds with bond order of 1.99 and the ground state can be described formally as a closed-shell singlet of [CeCSc 2 ] 6+ @[C 80 ] 6– , which is consistent with the typical [C 80 ] 6 – charge state. The carbon cage plays a crucial role in stabilization and protection of CeCSc 2 cluster and the stronger bonding between carbon and cerium compared to scandium in the CeCSc 2 cluster leads to the formation of the novel Ce ≡ C triple bond, along with the Sc–C–Sc three-center two-electron bond. This work uncovers the surprising ability of lanthanides to form stable, triple-bonded compound, thereby revitalizing the lanthanide bonding theory and paving way for new adventure in synthetic chemistry and application fields. Declarations Competing interests The authors declare that they have no competing interests. Author contributions N.C. and J.L. administrated the project; N.C. and J.L. supervised the project; H.J and M.G. performed the synthesis and isolation. J.Z. performed the DFT calculations; H.J. grew the single crystals and Q.M. solved the crystallographic data. H.J. carried out the Vis-IR and IR characterizations. H.J., J.Z., X.-K. Z., H.-S. H., J. L. and N. C. co-wrote the manuscript. All authors discussed the results and commented on the manuscript. Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC Nos. 22033005 and 52172051), the NSFC Center for Single-Atom Catalysis (22388102), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Guangdong Provincial Key Laboratory of Catalysis (No. 2020B121201002). Data availability The crystallographic data of CeCSc 2 @C 80 can be obtained free of charge from the Cambridge Crystallographic Data Centre with CCDC number 2346385, respectively, via https://www.ccdc.cam.ac.uk/data_request/cif . The data that support the findings of this study are available from the corresponding authors upon reasonable request. References Lewis, G. N. The Atom and the Molecule. J. Am. Chem. Soc. 38, 762–785 (1916). Nugent, W. A. & Mayer, J. M. Metal-Ligand Multiple Bonds: The Chemistry of Transition Metal Complexes Containing Oxo,Nitrido, Imido, Alkylidene, or Alkylidyne Ligands . (Wiley, New York, 1988). Ortu, F. Rare earth starting materials and methodologies for synthetic chemistry. Chem. Rev. 122, 6040–6116 (2022). Zepf, V. Rare Earth Elements: A New Approach to the Nexus of Supply, Demand and Use: Exemplified along the Use of Neodymium in Permanent Magnets . (Springer, Berlin, Heidelberg, 2013). Wall, F. 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Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 77, 3865–3868 (1996). Stephens, P. J., Devlin, F. J., Chabalowski, C. F. & Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 98, 11623–11627 (1994). Adamo, C. & Barone, V. Toward reliable density functional methods without adjustable parameters: The PBE0 model. J. Chem. Phys. 110, 6158–6170 (1999). Reed, A. E. & Weinhold, F. Natural localized molecular orbitals. J. Chem. Phys. 83, 1736–1740 (1985). Zhang, J.-X., Sheong, F. K. & Lin, Z. Unravelling Chemical Interactions with Principal Interacting Orbital Analysis. Chem. Eur. J. 24, 9639–9650 (2018). Zubarev, D. Y. & Boldyrev, A. I. Developing paradigms of chemical bonding: adaptive natural density partitioning. PCCP 10, 5207–5217 (2008). Baker, C. F., Seed, J. A., Adams, R. W., Lee, D. & Liddle, S. T. 13 C carbene nuclear magnetic resonance chemical shift analysis confirms Ce IV =C double bonding in cerium(IV)–diphosphonioalkylidene complexes. Chem. Sci. 15, 238–249 (2024). Michalak, A., DeKock, R. L. & Ziegler, T. Bond Multiplicity in Transition-Metal Complexes: Applications of Two-Electron Valence Indices. J. Phys. Chem. A 112, 7256–7263 (2008). Glendening, E. D. & Weinhold, F. Natural resonance theory: I. General formalism. J. Comput. Chem. 19, 593–609 (1998). Bader, R. F. W. A quantum theory of molecular structure and its applications. Chem. Rev. 91, 893–928 (1991). Su, W. et al. Cerium–carbon dative interactions supported by carbodiphosphorane. Dalton Trans. 48, 16108–16114 (2019). Kaoru Kobayashi, Shigeru Nagase, & Takeshi Akasaka. A theoretical study of C 80 and La 2 @C 80 . Chem. Phys. Lett. 245, 230–236 (1995). Bickelhaupt, F. M. & Baerends, E. J. Kohn-sham density functional theory: predicting and understanding chemistry. in Reviews in Computational Chemistry 1–86 (John Wiley & Sons, Ltd, 2000). doi: 10.1002/9780470125922.ch1 . Dolg, M., Stoll, H. & Preuss, H. A combination of quasirelativistic pseudopotential and ligand field calculations for lanthanoid compounds. Theor. Chim. Acta 85, 441–450 (1993). Tables Table 1 Geometric parameters of CeCSc 2 in C 80 and in aromatic ligand coordination a . Species Bond Length (Å) Dihedral Angle (°) L b −Ce Ce ≡ C C − Sc1/ C − Sc2 Sc1 − L/ Sc2 − L Sc1 − C−Sc2 − Ce CeCSc 2 @C 80 (Expt.) 2.551 1.978 2.010/2.035 2.407/2.433 172.4 CeCSc 2 @C 80 (Cal.) 2.590 2.049 2.015/2.016 2.412/20418 176.9 (C 5 H 5 )CeC[Sc(C 8 H 8 )] 2 + (Cal.) 2.754 1.968 2.170/2.128 2.378/2.397 173.2 a Bond lengths (in Å, average values are italic) and bond angles (in o ) are from X-ray diffraction of CeCSc 2 @C 80 ·[Ni-OEP] co-crystals and the computed data are from B3LYP/6-31G*+SDD calculation. b In CeCSc 2 @C 80 , L for Ce is the nearest benzene portion, for two Sc are the the nearest eight carbon atoms, including a benzene ring part and the two atoms next to it. In (C 5 H 5 )CeC[Sc(C 8 H 8 )] 2 + , L are the nearest carbon rings to the corresponding metal. Table 2 Atomic orbitals (AOs) contributions of NLMOs in CeCSc 2 @C 80 at B3LYP/6-31G*+SDD. Bond AOs% contributions Ce 4f/5d/6s C 2s/2p Sc 3d/4s Ce–C σ 28 57/39/3 64 29/71 6 96/1 Ce–Cπ || 19 40/60/0 72 0/100 8 93/6 Ce–C π ⊥ 17 42/58/0 69 0/100 13 97/0 Sc–C–Sc σ 4 13/61/25 83 72/28 12 64/35 Methods Materials Reagents used for synthesis were: Sc 2 O 3 (99.9%, J&K Scientific); CeO 2 (99.9%, Aladdin); Graphite rod (99.9%, Shanghai Fengyi Carbon Corporation); Graphite powder (99.9%, Shanghai Fengyi Carbon Corporation); Carbon disulfide (HPLC ≥99.9%, Aladdin); Acetone (AR, Chinasun Specialty Products Co., Ltd.); Toluene (AR, Aladdin); n -Hexane (HPLC ≥99.9%, Rhawn); Methanol (HPLC ≥99.9%, Meryer); Benzene (99.8%, Sigma-Aldrich); He (99.999%, purchased from Jinhong Gas). Synthesis and Isolation CeCSc 2 @C 80 was synthesized by a modified Krätschmer-Huffman direct-current arc-discharge method. The graphite rods, packed with CeO 2 , Sc 2 O 3 and graphite power (molar ratio of Ce/Sc/C = 1: 1: 24), were vaporized in a VDK250 arcing reactor (Beijing Technol Science Co., Ltd., China) under a 110 A direct current and 200 Torr He atmosphere. The targeted Ce-based metallofullerenes were extracted from the carbon soot by CS 2 for 24 hours. This solution was then filtered through a sand core suction filtration device and the obtained filtrate was spin-dried with a rotary evaporator, then dissolved in toluene to obtain a crude extract. The crude extract was isolated using an LC-9230II NEXT recycling preparative HPLC machine (Japan Analytical Industry Co., Ltd., Japan). Four types of Cosmosil columns (Nacalai Tesque Inc., Japan), including a preparative Buckyprep-M (20 × 250 mm), a semi-preparative Buckprep (10 × 250 mm), a semi-preparative 5PYE (10 × 250 mm), and a semi-preparative 5PBB (10 × 250 mm), were utilized in the HPLC procedures. Mass spectroscopy The positive-ion mode matrix-assisted laser desorption/ionization time-of-flight (Bruker, Germany) was employed for the mass characterization. Visible-near-infrared (Vis-NIR) absorption spectroscopy The UV−vis−NIR spectrum of the purified CeCSc 2 @C 80 was measured in CS2 solution with a Cary 5000 UV−vis−NIR spectrophotometer (Agilent, U.S.). Fourier transform infrared (FTIR) absorption spectroscopy The Micro Fourier transform infrared spectra were measured at room temperature by a Vertex 70 + Hyperon 1000 spectrometer (Bruker, Germany) with a resolution of 4 cm −1 and signal to noise ratio (7000:1). Single-crystal X-ray diffraction (SC-XRD) The black block co-crystals of CeCSc 2 @C 80 ·[Ni II -OEP] 55 (OEP = 2, 3, 7, 8, 12, 13, 17, 18-octaethylporphyrin dianion) was obtained by slow diffusion of a benzene solution of Ni II -OEP into a carbon disulfide solution of the metallofullerene sample. Single-crystal X-ray data of CeCSc 2 @C 80 was collected at 120 K on a diffractometer (APEX II; Bruker D8 Venture) equipped with a CCD collector. The multi-scan method was used for absorption correction. The refinement of the crystallographic structures was performed in the Olex2 software 56 packed with SHELXL-2018 57 by the full-matrix least-squares method. Quantum-chemical and Computational Methods The calculations were done with ADF code in AMS22 58 . The PBE functional 41 was used along with the zero-order regular approximation (ZORA) 59–61 to account for the scalar relativistic (SR) effect. The spin-orbit (SO) coupling effect was not taken into account because it is expected not to have significant influence for these systems. Slater-type orbital (STO) basis sets 62 with small frozen core were utilized. Double-ζ polarized (DZP) basis sets were used for carbon atoms on the fullerene cage, while triple-ζ plus two polarization functions (TZ2P) were used for other atoms. Additionally, the same methods and non-frozen core TZ2P basis set were employed to compute the overlap integrals between Ce or Sc atoms and C atoms at experimental bond lengths. The singlet, triplet, quintet, and septet states of CeCSc 2 @C 80 were optimized separately. Further optimizations of the singlet and triplet states were also performed using the B3LYP 42 functional implemented in Gaussian16 63 and the PBE0 43 functional in ORCA 5.0 64 for comparison. For Gaussian16 calculations, 6-31G* basis sets 65 were applied for carbons, whereas the SDD pseudopotential and corresponding basis sets, namely ECP28MWB 66,67 and ECP10MDF 68,69 , were used for Sc and Ce, respectively. In ORCA calculations, the def2-SVP basis set 70 was applied to carbons on the fullerene, def2-TZ2P-PP 66,71 for Ce, and def2-TZVP for other elements. And orbital transmission via the MOKIT program 72 was employed to expedite convergence. Despite using various methods like spin-flip in AMS, the fragment guess in Gaussian, and the broken symmetry in ORCA, the calculated spin-polarized singlet state always converged to a closed shell singlet, indicating that close-shell singlet is indeed the likely ground state. In addition, the calculated spin density of the triplet state CeCSc 2 @C 80 is mainly located on the Ce atom and the fullerene cage, while the experimental electron paramagnetic resonance (EPR) spectra did not detect any free radical signals on the fullerene cage, further confirming the stability of the singlet state. The infrared spectrum obtained via ORCA was scaled using a correction factor of 0.9572, derived from the experimental frequency of CO at 2143 cm −1 . AMS program was used to provide Mayer bond orders 73 , Gopinathan-Jug (G-J) bond orders 74 , and Nalewajski-Mrozek #3 (N-M(3)) bond orders 48 . Natural Localized Molecular Orbital (NLMO) analysis 44 , Natural Resonance Theory (NRT) analysis 49,75 and Wiberg Bond Index (WBI) analysis 76 were performed using the NBO 6.0 program 77 . Additionally, Adaptive Natural Density Portioning (AdNDP),analysis 46 and ‘quantum theory of atoms-in-molecules’ (QTAIM) analysis 50 were conducted using the Multiwfn program 78 . References Olmstead, M. M. et al. Interaction of Curved and Flat Molecular Surfaces. The Structures of Crystalline Compounds Composed of Fullerene (C 60 , C 60 O, C 70 , and C 120 O) and Metal Octaethylporphyrin Units. J. Am. Chem. Soc. 121 , 7090–7097 (1999). Paquette, L. A. Dodecahedrane—The chemical transliteration of Plato’s universe (A Review). P. Natl. Acad. Sci. Usa. 79 , 4495–4500 (1982). George M. Sheldrick. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 71 , 3–8 (2015). Rüger, R. et al. AMS 2022.1, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands,. E. van Lenthe, E. J. Baerends & J. G. Snijders. Relativistic regular two-component Hamiltonians. J. Chem. Phys. 99 , 4597–4610 (1993). E. van Lenthe, E. J. Baerends & J. G. Snijders. Relativistic total energy using regular approximations. J. Chem. Phys. 101 , 9783–9792 (1994). van Lenthe, E., Ehlers, A. & Baerends, E.-J. Geometry optimizations in the zero order regular approximation for relativistic effects. J. Chem. Phys. 110 , 8943–8953 (1999). van Lenthe, E. & Baerends, E. J. Optimized Slater-type basis sets for the elements 1–118. J. Comput. Chem. 24 , 1142–1156 (2003). Frisch, M. J. et al. Gaussian 16 Rev. B.01. (2016). Neese, F. The ORCA program system. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2 , 73–78 (2012). Hariharan, P. C. & Pople, J. A. The influence of polarization functions on molecular orbital hydrogenation energies. Theor. Chim. Acta 28 , 213–222 (1973). M. Dolg, H. Stoll & H. Preuss. Energy-adjusted ab initio pseudopotentials for the rare earth elements. J. Chem. Phys. 90 , 1730–1734 (1989). Cao, X. & Dolg, M. Segmented contraction scheme for small-core lanthanide pseudopotential basis sets. J. Molec. Struct. (Theochem) 581 , 139–147 (2002). Dolg, M., Wedig, U., Stoll, H. & Preuss, H. Energy-adjusted ab initio pseudopotentials for the first row transition elements. J. Chem. Phys. 86 , 866–872 (1987). Martin, J. M. L. & Sundermann, A. Correlation consistent valence basis sets for use with the Stuttgart–Dresden–Bonn relativistic effective core potentials: The atoms Ga–Kr and In–Xe. J. Chem. Phys. 114 , 3408–3420 (2001). Weigend, F. & Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 7 , 3297–3305 (2005). Gulde, R., Pollak, P. & Weigend, F. Error-balanced segmented contracted basis sets of double-ζ to quadruple-ζ valence quality for the lanthanides. J. Chem. Theory Comput. 8 , 4062–4068 (2012). Zou, J. Molecular Orbital Kit (MOKIT). Mayer, I. Charge, bond order and valence in the AB initio SCF theory. Chem. Phys. Lett. 97 , 270–274 (1983). Gopinathan, M. S. & Jug, K. Valency. I. A quantum chemical definition and properties. Theor. Chim. Acta 63 , 497–509 (1983). Glendening, E. D., Badenhoop, J. K. & Weinhold, F. Natural resonance theory: III. Chemical applications. J. Comput. Chem. 19 , 628–646 (1998). Wiberg, K. B. Application of the pople-santry-segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane. Tetrahedron 24 , 1083–1096 (1968). Glendening, E. D., Landis, C. R. & Weinhold, F. NBO 6.0: Natural bond orbital analysis program. J. Comput. Chem. 34 , 1429–1437 (2013). Lu, T. & Chen, F. Multiwfn: A multifunctional wavefunction analyzer. J. Comput. Chem. 33 , 580–592 (2012). Additional Declarations There is NO Competing Interest. Supplementary Files TableofContent.docx CeCTriplebondSI.docx NCHEM24102921LiCeSc2C8117301828971.cif CeSc2C81.cif NCHEM24102921LiSI.docx Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 19 Jun, 2025 Read the published version in Nature Chemistry → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5351349","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":376869418,"identity":"3fdf0d10-8b9d-4c40-972e-5d941a6103f1","order_by":0,"name":"Jun Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5klEQVRIiWNgGAWjYBACxgYGxgMJFQwMbGAEAgcIa2E4kHDGgAQtYDWMbQYgmkgtzDNyDxx4OO+PPZ90+7XHPDUMcnw3Ehg/F+Bz2Iy8hAOJ2wwS22TOlBvzHGMwlryRwCw9A6+WHAOQlgQ2iZw0aR42hsQNNxLYmHkIapljYA/R8o+hnkgtDQaMbRLpx6R52xgSDAhq6XljcCDhmHFim0QOm+TcPgnDmWceNkvj02LYnmP48EeNnL38jPRnEm++2cjzHU8++BmvlgY4kwcUORIMkOjFA+QRTPYHeFWOglEwCkbByAUAIilKdXhVrGMAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-8456-3980","institution":"Tsinghua University","correspondingAuthor":true,"prefix":"","firstName":"Jun","middleName":"","lastName":"Li","suffix":""},{"id":376869419,"identity":"9da654d9-2990-42eb-9c3d-396547c8daf4","order_by":1,"name":"Ning Chen","email":"","orcid":"https://orcid.org/0000-0002-9405-6229","institution":"College of Chemistry, Chemical Engineering and Materials Science, and State Key Laboratory of Radiation Medicine and Protection, Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Ning","middleName":"","lastName":"Chen","suffix":""},{"id":376869420,"identity":"f921903f-b44a-47df-97b3-23b7b5ad7598","order_by":2,"name":"Hongjie Jiang","email":"","orcid":"","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Hongjie","middleName":"","lastName":"Jiang","suffix":""},{"id":376869421,"identity":"3e4d3a1b-2bc4-465d-b8cd-7446fc78c5b6","order_by":3,"name":"Jing Zhao","email":"","orcid":"","institution":"Department of Chemistry and Engineering Research Center of Advanced Rare-Earth Materials of the Ministry of Education, Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Jing","middleName":"","lastName":"Zhao","suffix":""},{"id":376869422,"identity":"c8452a16-e5ab-47e2-bdcd-c9960a59806a","order_by":4,"name":"Qingyu Meng","email":"","orcid":"","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Qingyu","middleName":"","lastName":"Meng","suffix":""},{"id":376869423,"identity":"bdc4a2de-dc8e-47c1-931d-5ace194dc5df","order_by":5,"name":"Xiao-Kun Zhao","email":"","orcid":"","institution":"Southern University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiao-Kun","middleName":"","lastName":"Zhao","suffix":""},{"id":376869424,"identity":"6156018c-6464-48c5-9282-71846c6feeb7","order_by":6,"name":"Min Guo","email":"","orcid":"","institution":"College of Chemistry, Chemical Engineering and Materials Science, and State Key Laboratory of Radiation Medicine and Protection, Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Min","middleName":"","lastName":"Guo","suffix":""},{"id":376869425,"identity":"bdb1b70e-1dbd-45e0-bad0-5915ded07208","order_by":7,"name":"Han-Shi Hu","email":"","orcid":"https://orcid.org/0000-0001-9508-1920","institution":"Tsinghua University","correspondingAuthor":false,"prefix":"","firstName":"Han-Shi","middleName":"","lastName":"Hu","suffix":""}],"badges":[],"createdAt":"2024-10-29 05:50:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5351349/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5351349/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41557-025-01856-2","type":"published","date":"2025-06-19T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":68883347,"identity":"c1cbe91c-786f-4cfd-8c17-0bac763887ad","added_by":"auto","created_at":"2024-11-13 06:22:18","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":87198,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOak Ridge thermal ellipsoid plots (ORTEP) of CeCSc\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e@C\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e80\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e predominant crystal conformations, drawn at 15% ellipsoids. (A)\u003c/strong\u003e CeCSc\u003csub\u003e2\u003c/sub\u003e cluster inside the fullerene cage above the co-crystallized Ni\u003csup\u003eII\u003c/sup\u003e(OEP) molecule;\u003cstrong\u003e \u003c/strong\u003eall solvent molecules and hydrogen atoms are omitted. \u003cstrong\u003e(B)\u003c/strong\u003e The interactions of the Ce and Sc atoms of CeCSc\u003csub\u003e2\u003c/sub\u003e cluster with the closest hexagonal aromatic ring fragments, interatomic distances in Å.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/4c071f7c995e545c779a3a58.png"},{"id":68882804,"identity":"c72d1ede-602d-43c8-b35e-c52418b9552a","added_by":"auto","created_at":"2024-11-13 06:14:18","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":36983,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eIR and Vis-NIR results of CeCSc\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e@C\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e80\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e. (A) \u003c/strong\u003eExperimental and simulated FTIR spectra of CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e.\u003cstrong\u003e (B)\u003c/strong\u003e Vis-NIR spectrum of CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e. The simulated FTIR spectra are from calculations at PBE0/def2-TZVP+def2-SVP level.\u0026nbsp;\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/9b6aebfc31dc072590136d70.png"},{"id":68882805,"identity":"b83e0239-3890-4f8d-a40e-275854cba5b4","added_by":"auto","created_at":"2024-11-13 06:14:18","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2620483,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(A) Schematic Kohn-Sham MO energy-level diagram of CeCSc\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e@C\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e80\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e; (B) Canonical MO contours of Ce≡C triple bonds.\u003c/strong\u003e The orbitals are from calculations at PBE/TZ2P+DZP level. The cutoff of the MO isosurfaces is 0.03 a.u.; Ce, C and Sc atoms are represented in orange, gray and pink respectively.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/a6285e4c3a0967e07953fd81.png"},{"id":68882801,"identity":"4dad1bd2-9eb7-4073-93e6-e6a097b4268f","added_by":"auto","created_at":"2024-11-13 06:14:18","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":95132,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eNatural localized molecular orbital (NLMO) analysis of CeCSc\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e@C\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e80\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e The orbitals are from calculations at B3LYP/6-31G*+SDD level. The cutoff of isosurfaces is ±0.05 a.u..\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/02f69f8c99ec0d1d18793f6f.png"},{"id":85029623,"identity":"656e9ad9-25a1-428c-81c6-8b31abfe26e3","added_by":"auto","created_at":"2025-06-20 07:07:29","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3739893,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/2613b6fe-47ba-43a0-a5a2-bb5f5a7f58fc.pdf"},{"id":68882803,"identity":"83edf199-759d-4c45-b0f7-7c8319766850","added_by":"auto","created_at":"2024-11-13 06:14:18","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":215181,"visible":true,"origin":"","legend":"","description":"","filename":"TableofContent.docx","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/2cf90b9344e7900f6bd19d46.docx"},{"id":68882807,"identity":"669b5a4c-8100-48b3-8019-384a1b65864e","added_by":"auto","created_at":"2024-11-13 06:14:18","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":2580118,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"CeCTriplebondSI.docx","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/f352b7fe0411937473f71171.docx"},{"id":68883348,"identity":"fce80393-c607-4957-9233-ab1b79f8659d","added_by":"auto","created_at":"2024-11-13 06:22:18","extension":"cif","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":2056476,"visible":true,"origin":"","legend":"\u003cp\u003eCeSc2C81.cif\u003c/p\u003e","description":"","filename":"NCHEM24102921LiCeSc2C8117301828971.cif","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/ef901cba83cf882da9e4c829.cif"},{"id":68883349,"identity":"41197ae5-56a6-47e0-9d9d-030df5c34a35","added_by":"auto","created_at":"2024-11-13 06:22:18","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":2668714,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Information\u003c/p\u003e","description":"","filename":"NCHEM24102921LiSI.docx","url":"https://assets-eu.researchsquare.com/files/rs-5351349/v1/f0f8ae7971070c7e3fa0253d.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Endeavoring the First Lanthanide–Carbon Triple-Bond in Fullerene Cage","fulltext":[{"header":"Introduction","content":"\u003cp\u003eChemical bonding is at the heart of chemistry\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Substantial progress has been achieved in understanding the chemical bonds of main-group and transition-metal elements over the decades, particularly through the synthesis of molecular compounds with metal\u0026ndash;ligand multiple bonds\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. However, lanthanides (Ln\u0026thinsp;=\u0026thinsp;La\u0026ndash;Lu) of the 4f series, which have significant industrial roles in catalysis, magnets, photonics, alloys, and energy\u003csup\u003e\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e, are viewed as less likely to form multiple bonds with ligands due to their rather contracted 4f orbitals and limited radial extension of valence orbitals\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Lanthanides exhibit fairly uniform behavior throughout the series, mainly forming trivalent cations that are characterized by ionic and highly polarized bonding\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Moreover, the energy levels of lanthanide 5d valence orbitals are notably higher than those of transition metals, making lanthanide metal\u0026ndash;ligand multiple bonds highly reactive and extremely difficult to be stabilized due to their low covalency and strong charge polarization\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDespite these challenges, significant efforts are made to the synthesis of complexes with lanthanide\u0026ndash;ligand multiple bonds. Over the past two decades, several successful syntheses of alkylidenes\u003csup\u003e\u003cspan additionalcitationids=\"CR11 CR12 CR13 CR14\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, alkylidynes\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, imido\u003csup\u003e\u003cspan additionalcitationids=\"CR19 CR20 CR21 CR22\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, and oxo\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e compounds have contributed to the understanding of lanthanide\u0026ndash;ligand multiple bonding. However, due to their high Lewis acidity, oxophilicity, and large coordination spheres, the multiple bonds formed between lanthanides and carbon are generally highly polarized and possess low covalency and bond order\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Cerium, which can be stabilized in a tetravalent oxidation state (Ce\u003csup\u003eIV\u003c/sup\u003e), has been considered a promising candidate among lanthanides for forming lanthanide\u0026ndash;ligand multiple bonding\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. However, theoretical and experimental investigations regarding Ce\u0026thinsp;=\u0026thinsp;C double bonds and the role of 4f orbitals in bonding remain controversial. Previous theoretical calculations suggested a Ce\u0026thinsp;=\u0026thinsp;C bond length of 2.127 \u0026Aring; in Cp\u003csub\u003e2\u003c/sub\u003eCeCH\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e28\u003c/sup\u003e, which was assumed to be in a closed-shell singlet state. The Ce\u0026thinsp;=\u0026thinsp;C double bond distance of 2.042 \u0026Aring; was predicted for CeCH\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e cation\u003csup\u003e29\u003c/sup\u003e. However, more comprehensive theoretical studies have indicated that Ce and methylene do not form terminal double bonds but rather generate a \u0026ldquo;substituted methyl radical\u0026rdquo; H\u003csub\u003e2\u003c/sub\u003eC\u0026bull;\u0026minus;\u0026bull;CeL\u003csub\u003e2\u003c/sub\u003e (L\u0026thinsp;=\u0026thinsp;F\u003csup\u003e30,31\u003c/sup\u003e, Cp\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e), exhibiting characteristics of a non-Lewis paired diradical species as demonstrated in U\u0026ndash;Si complexes with so-called multi-radical bonding (MRB)\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. This kind of open-shell singlet character of H\u003csub\u003e2\u003c/sub\u003eC\u0026bull;\u0026minus;\u0026bull;CeF\u003csub\u003e2\u003c/sub\u003e complex\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e might provide a hint at the repeated unsuccess in synthesizing compounds with terminal Ce\u0026thinsp;=\u0026thinsp;C double bonds. To date, only two compounds featuring predominantly electrostatic Ce\u0026thinsp;=\u0026thinsp;C double bonds have been reported, with bond lengths of 2.441 \u0026Aring; in [Ce(BIPM\u003csup\u003eTMS\u003c/sup\u003e)(ODipp)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e and 2.385 \u0026Aring; in [Ce(BIPM\u003csup\u003eTMS\u003c/sup\u003e)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. These bond distances are only slightly shorter than the s-single-bond distances in the recently synthesized Ce(IV)-alkyl, -aryl, and -alkynyl complexes, where the Ce\u003csup\u003eIV\u003c/sup\u003e\u0026ndash;C σ-bond distances lie between 2.48\u0026ndash;2.54 \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWhile lanthanide metal\u0026ndash;carbon terminal multiple bonds with higher covalency had yet to be achieved, Ln\u0026equiv;C triple bonds, which seem to be beyond the bonding ability of 4f elements, appear unlikely to be formed. The effort towards preparing compounds with actinide triple bonds had been unsuccessful. However, our studies suggest that fullerenes can act as nano containers and effectively stabilize bonding motifs which otherwise are not available in conventional compounds. We have designed the strategy by making use of fullerene confinement to isolate U\u0026equiv;C bonds\u003csup\u003e\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. These results further inspired us to explore the possibility of stabilizing Ln\u0026ndash;C multiple bonding, which can provide insights into the fundamental bonding nature of lanthanides. Herein, we report the synthesis and characterization of the endohedral metallofullerene (EMF) compound, CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e, which contains a unique Ce\u0026equiv;C triple bond. Through X-ray crystallographic analysis, the structural characterization unveils a Ce\u0026equiv;C bond of 1.978(8) \u0026Aring;, which is the shortest ever recorded to date. The nature of the Ce\u0026equiv;C triple bond has been investigated by spectroscopic methods and quantum-theoretical analyses.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis and Crystallographic Characterization of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eCeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e was synthesized by a modified Kr\u0026auml;tschmer-Huffman direct-current arc-discharge method\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. The graphite rods, packed with CeO\u003csub\u003e2\u003c/sub\u003e, Sc\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and graphite powder (molar ratio of Ce:Sc:C\u0026thinsp;=\u0026thinsp;1:1:24), were vaporized in an arcing reactor under a 110 A direct current and 200 Torr He atmosphere. The targeted Ce-based metallofullerene was extracted from the carbon soot by CS\u003csub\u003e2\u003c/sub\u003e solvent for 12 hours and then isolated by multi-step high-performance liquid chromatography (HPLC) (Supplementary Fig.\u0026nbsp;1). The high purity of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e was confirmed by chromatograms and mass spectra (Supplementary Fig.\u0026nbsp;2). The experimental isotopic distributions of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e agree well with the theoretical simulation, confirming its elemental composition.\u003c/p\u003e \u003cp\u003eBlack block cocrystals of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e via strong π-π interactions with Ni\u003csup\u003eII\u003c/sup\u003e(OEP) (OEP\u0026thinsp;=\u0026thinsp;2, 3, 7, 8, 12, 13, 17, 18-octa-ethyl-porphyrin dianion) were obtained by the slow diffusion of a benzene solution of [Ni\u003csup\u003eII\u003c/sup\u003e(OEP)] into a CS\u003csub\u003e2\u003c/sub\u003e solution of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e. The molecular structure was solved in the monoclinic space group \u003cem\u003eC\u003c/em\u003e2/\u003cem\u003em\u003c/em\u003e and determined to be CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e by single-crystal X-ray diffraction analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA and Supplementary Table\u0026nbsp;1). CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e has two orientations of carbon cages with the same occupancy of 0.5, due to the crystallographic mirror plane in the \u003cem\u003eC\u003c/em\u003e2\u003cem\u003e/m\u003c/em\u003e space group. As to the inner cluster, the central carbide atom, C81, is situated on the mirror plane and fully ordered with an occupancy of 0.5. There are three crystallographically disordered sites for Ce atoms (Ce1, Ce2, and Ce3) and four sites for Sc atoms (Sc1, Sc2, Sc3, and Sc4). Additionally, due to the presence of crystallographic mirror planes, mirror-symmetrical atoms are generated correspondingly (Supplementary Table\u0026nbsp;2). Among these, the major metal sites for Ce atoms are at Ce1, with an occupancy of 0.4035(15), while for Sc atoms, they are at Sc1 and Sc2, with corresponding occupancies of 0.435(4) and 0.324(7), respectively. Hence, the inner cluster exhibits relatively high occupancy, ensuring the accuracy of subsequent analysis of the cluster structure and chemical bonding in CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eIn CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e, the distances between two Sc atoms and the nearest carbon atoms on the cage are 2.166 \u0026Aring; and 2.205 \u0026Aring;, respectively. The distances between Ce and the carbon atoms in the adjacent hexagonal ring range from 2.478 \u0026Aring; to 2.619 \u0026Aring;, which are within the Ce\u0026ndash;C single bond distance. Overall, the CeCSc\u003csub\u003e2\u003c/sub\u003e cluster nearly forms a planar triangle with a total bond angle of 359.6\u0026deg; (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB), and the distances between C81 and Sc atoms in the cluster are 2.010(7) \u0026Aring; and 2.035(9) \u0026Aring;, respectively. Furthermore, the interatomic distances and angles, including the bond lengths of C\u003csub\u003ecage\u003c/sub\u003e\u0026minus;Ce, Sc\u0026thinsp;\u0026minus;\u0026thinsp;C\u003csub\u003ecage\u003c/sub\u003e, C81\u0026thinsp;\u0026minus;\u0026thinsp;Sc, and the Sc1\u0026thinsp;\u0026minus;\u0026thinsp;C81\u0026thinsp;\u0026minus;\u0026thinsp;Sc2\u0026thinsp;\u0026minus;\u0026thinsp;Ce dihedral angles in the theoretically optimized spin-singlet CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e molecules (\u003cem\u003evide infra\u003c/em\u003e), all fit well with the experimental values (Supplementary Tables\u0026nbsp;3\u0026ndash;4).\u003c/p\u003e \u003cp\u003eThe most striking feature of the molecular structure of this fullerene compound is the very short Ce\u0026ndash;C bond distance measured as 1.978(8) \u0026Aring;, the shortest cerium\u0026ndash;carbon bond distance ever reported in known compounds. This bond distance is shorter not only than the theoretical distance of 2.4 \u0026Aring; for a Ce\u0026thinsp;\u0026minus;\u0026thinsp;C single bond\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e, suggesting its multiple bonding character, but also shorter than the theoretical distances of 2.127 \u0026Aring; for C\u0026thinsp;=\u0026thinsp;Ce in Cp\u003csub\u003e2\u003c/sub\u003eCe(CH\u003csub\u003e2\u003c/sub\u003e)\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e and 2.042 \u0026Aring; for Ce\u0026thinsp;=\u0026thinsp;C terminal double bonds in CeCH\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e cation\u003csup\u003e29\u003c/sup\u003e, as well as the sum of Ce and C double-bond covalent radii of 2.04 \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e, indicating an even higher bond order. Furthermore, compared to the experimentally reported Ce\u0026thinsp;=\u0026thinsp;C bond distances of 2.441 \u0026Aring; in [Ce(BIPM\u003csup\u003eTMS\u003c/sup\u003e)(ODipp)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e and 2.385 \u0026Aring; in [Ce(BIPM\u003csup\u003eTMS\u003c/sup\u003e)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, the remarkably short cerium\u0026ndash;carbon bond length of 1.978(8) \u0026Aring; in this compound appears to imply a significantly stronger interaction between Ce and C atoms. Additionally, the experimentally determined distances agree well with the Ce\u0026thinsp;\u0026minus;\u0026thinsp;C81 bond lengths computationally optimized for CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Supplementary Table\u0026nbsp;4). Altogether, these results provide evidences for the presence of a cerium\u0026ndash;carbon bond with high bond order in a stable molecular compound.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSpectroscopic Properties\u003c/h3\u003e\n\u003cp\u003eFourier Transform Infrared (FTIR) absorption spectroscopy and Visible-Near-Infrared (Vis-NIR) absorption spectroscopy were utilized to support the structural parameters of CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e and investigate the bonding nature of the encapsulated CeCSc\u003csub\u003e2\u003c/sub\u003e unit. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA, the experimental FTIR absorption spectrum of CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e agrees well with the corresponding calculated spectrum. In the high wavenumber range of 1600\u0026thinsp;~\u0026thinsp;800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, dominant stretching vibrations of the \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e carbon cage are observed. Characteristic vibrational peaks at 1520, 1383, and 1201 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the experimental spectrum correspond well with those theoretically simulated at 1529, 1395, and 1197 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. These features are consistent with the characteristic vibration peaks attributed to the carbon cage vibrations reported in previous studies\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. In addition, the endohedral CeCSc\u003csub\u003e2\u003c/sub\u003e unit contributes 12 modes, 10 of which are very weak due to the heavy metal. These modes include three translational rocking modes, three rotational modes and three Sc1\u0026ndash;C\u0026ndash;Sc2 vibration modes (in-plane scissoring, in-plane rocking and out-of-plane wagging) are located between approximately 20\u0026ndash;210 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, along with one Sc\u0026ndash;C\u0026ndash;Sc symmetric stretching mod at 697 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The remaining two more intense peaks at 735 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 696 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the experimental spectrum correspond to Ce\u0026ndash;C stretching modes and Sc\u0026ndash;C\u0026ndash;Sc asymmetric stretching, which align well with the theoretically simulated vibrations at 730 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 698 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe Vis-NIR absorption spectra of endohedral fullerenes are primarily dominated by π\u0026rarr;π* transitions within the large π system formed by the carbon atoms on the fullerene cage. Hence, the cage isomerism, the charge transfer, and interactions between the fullerene cage and the encapsulated species significantly affect the Vis-NIR absorption spectra of EMFs. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB, the absorption spectrum of CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e appears relatively smooth without significant characteristic absorption frequency. This absorption spectrum is very similar to the absorption of previously reported EMF with \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e symmetry carbon cages and six cluster-cage six electron transfer\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Accordingly, it appears reasonable to assume that the electronic configuration of CeCSc\u003csub\u003e2\u003c/sub\u003e@\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e(7)-C\u003csub\u003e80\u003c/sub\u003e is formally [CeCSc\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e6+\u003c/sup\u003e@[C\u003csub\u003e80\u003c/sub\u003e]\u003csup\u003e6\u0026minus;\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eElectron Structure and Bonding Analysis\u003c/h3\u003e\n\u003cp\u003eQuantum chemical studies using density functional theory (DFT) were conducted to validate the experimental findings and understand the electronic structures of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e. The geometric optimizations with PBE\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e, B3LYP\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e and PBE0\u003csup\u003e43\u003c/sup\u003e functional all reveal that the closed-shell singlet state, \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eA CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e, is the most stable configuration, with an equivalent chiral isomer \u003csup\u003e1\u003c/sup\u003eA* (Supplementary Table\u0026nbsp;5 and Supplementary Fig.\u0026nbsp;3). This singlet state is 13\u0026ndash;27 kcal/mol lower in energy than the second-most stable triplet state with different DFT functional. The calculated geometric parameters show fair agreement with experimental data, with the calculated Ce\u0026ndash;C bond length of 2.049 \u0026Aring; close to the experimental value of 1.978 \u0026Aring; (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) at the B3LYP level. With inclusion of dispersion description of B3LYP-D3BJ, the Ce\u0026ndash;C bond length reduces to 2.036 \u0026Aring;.\u003c/p\u003e \u003cp\u003eTheoretical bonding analyses reveal that a triple bond forms between Ce and the central C atom in the encapsulated [CeCSc\u003csub\u003e2\u003c/sub\u003e] unit. The Kohn-Sham energy level diagram of the frontier canonical molecular orbitals (MOs) and corresponding s/p MOs of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, show that the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond consists of one σ bond (MO 175) and two π bonds (MO 172\u0026ndash;173). The details are given in Supplementary Table\u0026nbsp;6 and Supplementary Fig.\u0026nbsp;4. This Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond is primarily contributed by C 2p orbitals, with minor contributions from Ce 4f/5d orbitals and nominal contributions from Sc 3d orbitals. The Natural Localized Molecular Orbital (NLMO) analysis\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e, the principal interacting orbital (PIO) analysis (Supplementary Fig.\u0026nbsp;5)\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e, and the Adaptive Natural Density Partitioning (AdNDP) analysis(Supplementary Fig.\u0026nbsp;6)\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e at the B3LYP level all confirm the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bonds.\u003c/p\u003e \u003cp\u003eFrom NLMO analysis shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond within CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e has Ce atom contributing 28% to the σ bond, and 19% and 17% to the two π bonds. The detailed atomic orbitals (AOs) contributions are provided in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. These contributions are significantly higher than the 6\u0026ndash;14% reported for Ce\u0026thinsp;=\u0026thinsp;C double dative bond in [Ce(BIPM\u003csup\u003eTMS\u003c/sup\u003e)(ODipp)\u003csub\u003e2\u003c/sub\u003e] and [Ce(BIPM\u003csup\u003eTMS\u003c/sup\u003e)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e, indicating enhanced covalency in the triple bond. Moreover, the calculated Nalewajske-Mrozek (N-M) bond order\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e of 1.99 for Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C is substantially higher than those (1.10\u0026ndash;1.16) for the reported Ce\u0026thinsp;=\u0026thinsp;C double bonds\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Usually, the bond orders (Supplementary Table\u0026nbsp;7) are smaller than the formal bond order due to significant bond polarity. Especially noteworthy is the results of Natural Resonance Theory (NRT) analysis\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e, which shows that the predominant resonance structure is the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple-bond component with a contribution of 60%, and the Ce\u0026thinsp;=\u0026thinsp;C double-bond component accounts for 30% (Supplementary Fig.\u0026nbsp;7).\u003c/p\u003e \u003cp\u003eThe quantum theory of atoms-in-molecules (QTAIM) analysis\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e provides a topological perspective to demonstrate the covalent character of the triple Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C bond in CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e compared to other reported compounds with Ce\u0026ndash;C bonds (Supplementary Table\u0026nbsp;8)\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. The electron density (\u003cem\u003eρ\u003c/em\u003e), Laplacian of electron density (\u0026nabla;\u0026sup2;\u003cem\u003eρ\u003c/em\u003e), electron energy density (\u003cem\u003eH\u003c/em\u003e), and delocalization index (DI) at the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C bond critical point (BCP) indicate stronger interactions and increased covalency between Ce and C atoms. The largest \u003cem\u003eρ\u003c/em\u003e of 0.16 and the most negative \u003cem\u003eH\u003c/em\u003e of \u0026minus;\u0026thinsp;0.09 at the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C BCP highlight strong covalent interaction. However, the positive \u0026nabla;\u0026sup2;\u003cem\u003eρ\u003c/em\u003e of 0.16 combined with an \u003cem\u003eη\u003c/em\u003e index of 0.44 indicates the polarity of this bond, consistent with the atomic contributions in the canonical or localized orbitals analysis. Additionally, both single and triple bonds typically exhibit cylindrical electron densities around the nuclear bonding axis, resulting in the BCP ellipticity (\u003cem\u003eε\u003c/em\u003e) value near zero. The \u003cem\u003eε\u003c/em\u003e for the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C bond in CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e is 0.12, corresponding to the value of a triple bond, and is significantly smaller than the ε value range of 0.32\u0026ndash;0.41 for Ce\u0026thinsp;=\u0026thinsp;C bonds.\u003c/p\u003e \u003cp\u003eWe have further evaluated the confinement effect of the [CeCSc\u003csub\u003e2\u003c/sub\u003e] cluster by the fullerene cage. It is found that the fullerene cage plays a crucial role in charge transfer, coordination interactions, electrostatic confinement, and spatial protection during the formation of the trimetallic carbon cluster with a Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond. It turns out that the [CeCSc\u003csub\u003e2\u003c/sub\u003e] cluster donates six electrons to C\u003csub\u003e80\u003c/sub\u003e, leading to a valence state assignment of [CeCSc\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e6+\u003c/sup\u003e@[C\u003csub\u003e80\u003c/sub\u003e]\u003csup\u003e6\u0026ndash;\u003c/sup\u003e, because the fourfold degenerate HOMOs (MO 160\u0026ndash;163) of \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e-C\u003csub\u003e80\u003c/sub\u003e only have two electrons, thus requiring six more electrons to become stable close-shell (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e Furthermore, the central C atom in [CeCSc\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e6+\u003c/sup\u003e accepts four electrons from the connected metal atoms, resulting in formal oxidation states of Sc\u003csup\u003e+\u0026thinsp;III\u003c/sup\u003e and Ce\u003csup\u003e+\u0026thinsp;IV\u003c/sup\u003e. To stabilize these high oxidation states, both covalent and ionic interactions between the [CeCSc\u003csub\u003e2\u003c/sub\u003e] unit and the fullerene cage are essential. The central part of [C\u003csub\u003e80\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003c/sup\u003e has a strong electrostatic potential (Supplementary Fig.\u0026nbsp;8), providing effective electrostatic confinement for the [CeCSc\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e6+\u003c/sup\u003e cation. The Ce 5d orbitals and the Sc 3d orbitals show significant coordination interactions with fullerenes, as demonstrated by PIO analysis (Supplementary Fig.\u0026nbsp;9). Energy decomposition analysis (EDA)\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e indicates that the interaction between [CeCSC\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e6+\u003c/sup\u003e and [C\u003csub\u003e80\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003c/sup\u003e fragments is predominantly electrostatic (68%), with orbital interactions contributing 32% (Supplementary Table\u0026nbsp;9). Additionally, the fullerene cage provides a matrix-like environment that spatially protects the [CeCSc\u003csub\u003e2\u003c/sub\u003e] species from redox lability. Replacing the rigid fullerene cage [C\u003csub\u003e80\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003c/sup\u003e with flexible carbon rings capable of accepting electrons does not significantly elongate the Ce\u0026ndash;C bond (Supplementary Table\u0026nbsp;10), indicating that the very short Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C bond distance is an intrinsic characteristic of the [CeCSc\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e6+\u003c/sup\u003e motif in the appropriate fullerene size.\u003c/p\u003e \u003cp\u003eThe formation of the counter-intuitive C\u0026thinsp;\u0026equiv;\u0026thinsp;Ce triple bond, along with the Sc\u0026ndash;C\u0026ndash;Sc three-center, two-electron bond, primarily arises from the stronger bonding affinity between carbon and cerium compared to scandium in both electrostatic and orbital interactions. Ce\u003csup\u003e+\u0026thinsp;IV\u003c/sup\u003e is more positively charged than Sc\u003csup\u003e+\u0026thinsp;III\u003c/sup\u003e (Supplementary Table\u0026nbsp;11), resulting in a stronger ionic interaction with C\u003csup\u003e\u0026ndash;IV\u003c/sup\u003e. Moreover, the overlap between Sc 3d AOs and C 2s2p AOs is less effective than that of Ce 5d AOs, leading to a weaker covalent interaction (Supplementary Fig.\u0026nbsp;10 and Supplementary Table\u0026nbsp;12).\u003c/p\u003e \u003cp\u003eThe degree of involvement of Ce 4f orbitals in rare-earth covalent bonding has been long debated. Inasmuch as 4f orbitals are extremely contracted in spatial extension,\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e they can hardly contribute to covalent bonding in rare earth compounds. Indeed, when optimizing the structure of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e using a 4f-in-core pseudopotential\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e and corresponding basis set, the structure remains largely unchanged, implying that the role of 4f orbitals is not significant. As Ce 4f and C2s/2p orbitals can hardly overlap at the experimental Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C distance of 1.98 \u0026Aring; (Supplementary Fig.\u0026nbsp;10), the involvement of Ce 4f orbitals in Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C bonds is much less than Ce 5d orbitals (Supplementary Table\u0026nbsp;12). At this very short Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C distance, the Ce 4f orbitals start to play a marginal role, albeit non-negligible, in the short multiple bonds between carbon and cerium in this EMF system.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, the synthesis, isolation and characterization of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e are reported herein. This endohedral metallofullerene is shown to feature hereto the first Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond. Solid-state characterization by X-ray crystallographic analysis reveals an exceptionally short cerium-carbon bond of 1.978(8) \u0026Aring;, which is notably shorter than the distances of experimentally reported Ce\u0026ndash;C single bonds and Ce\u0026thinsp;=\u0026thinsp;C double bonds distances and is close to the sum of Pyykk\u0026ouml; triple-bond covalent radii of 1.91 \u0026Aring;. The computational geometric parameters agree well with the experimental data, confirming the formation of a Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond. The quantum chemical investigations on bonding nature show that the Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond consists of one σ bond and two π bonds with bond order of 1.99 and the ground state can be described formally as a closed-shell singlet of [CeCSc\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e6+\u003c/sup\u003e@[C\u003csub\u003e80\u003c/sub\u003e]\u003csup\u003e6\u0026ndash;\u003c/sup\u003e, which is consistent with the typical [C\u003csub\u003e80\u003c/sub\u003e]\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003c/sup\u003e charge state. The carbon cage plays a crucial role in stabilization and protection of CeCSc\u003csub\u003e2\u003c/sub\u003e cluster and the stronger bonding between carbon and cerium compared to scandium in the CeCSc\u003csub\u003e2\u003c/sub\u003e cluster leads to the formation of the novel Ce\u0026thinsp;\u0026equiv;\u0026thinsp;C triple bond, along with the Sc\u0026ndash;C\u0026ndash;Sc three-center two-electron bond. This work uncovers the surprising ability of lanthanides to form stable, triple-bonded compound, thereby revitalizing the lanthanide bonding theory and paving way for new adventure in synthetic chemistry and application fields.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eN.C. and J.L. administrated the project; N.C. and J.L. supervised the project; H.J and M.G. performed the synthesis and isolation. J.Z. performed the DFT calculations; H.J. grew the single crystals and Q.M. solved the crystallographic data. H.J. carried out the Vis-IR and IR characterizations. H.J., J.Z., X.-K. Z., H.-S. H., J. L. and N. C. co-wrote the manuscript. All authors discussed the results and commented on the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work is supported by the National Natural Science Foundation of China (NSFC Nos. 22033005 and 52172051), the NSFC Center for Single-Atom Catalysis (22388102), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Guangdong Provincial Key Laboratory of Catalysis (No. 2020B121201002).\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe crystallographic data of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e can be obtained free of charge from the Cambridge Crystallographic Data Centre with CCDC number 2346385, respectively, via \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ccdc.cam.ac.uk/data_request/cif\u003c/span\u003e\u003cspan address=\"https://www.ccdc.cam.ac.uk/data_request/cif\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. The data that support the findings of this study are available from the corresponding authors upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLewis, G. N. The Atom and the Molecule. J. Am. Chem. Soc. 38, 762\u0026ndash;785 (1916).\u003c/li\u003e\n\u003cli\u003eNugent, W. A. \u0026amp; Mayer, J. M. \u003cem\u003eMetal-Ligand Multiple Bonds: The Chemistry of Transition Metal Complexes Containing Oxo,Nitrido, Imido, Alkylidene, or Alkylidyne Ligands\u003c/em\u003e. (Wiley, New York, 1988).\u003c/li\u003e\n\u003cli\u003eOrtu, F. Rare earth starting materials and methodologies for synthetic chemistry. Chem. Rev. 122, 6040\u0026ndash;6116 (2022).\u003c/li\u003e\n\u003cli\u003eZepf, V. \u003cem\u003eRare Earth Elements: A New Approach to the Nexus of Supply, Demand and Use: Exemplified along the Use of Neodymium in Permanent Magnets\u003c/em\u003e. (Springer, Berlin, Heidelberg, 2013).\u003c/li\u003e\n\u003cli\u003eWall, F. 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Multi-reference character and Ce 4\u003cem\u003ef\u003c/em\u003e orbital contributions in terminal multiple Ce\u0026thinsp;\u0026ndash;\u0026thinsp;Z bonds of Cp\u003csub\u003e2\u003c/sub\u003eCeZ (Z\u0026thinsp;=\u0026thinsp;CH\u003csub\u003e2\u003c/sub\u003e, CH\u003csup\u003e\u0026ndash;\u003c/sup\u003e, NH, O, F\u003csup\u003e+\u003c/sup\u003e) complexes. Comput. Theor. Chem. 1073, 34\u0026ndash;44 (2015).\u003c/li\u003e\n\u003cli\u003eHu, H.-S., Wei, F., Wang, X., Andrews, L. \u0026amp; Li, J. Actinide\u0026ndash;Silicon Multiradical Bonding: Infrared Spectra and Electronic Structures of the Si(\u0026micro;-X)AnF\u003csub\u003e3\u003c/sub\u003e (An =\u0026thinsp;Th, U; X\u0026thinsp;=\u0026thinsp;H, F) Molecules. J. Am. Chem. Soc. 136, 1427\u0026ndash;1437 (2014).\u003c/li\u003e\n\u003cli\u003eFeng, B. \u003cem\u003eet al.\u003c/em\u003e Cerium(IV)-Alkyl, -Aryl, and -Alkynyl Complexes Synthesized by an Energy-Level Match Strategy. CCS Chem. 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Acta 85, 441\u0026ndash;450 (1993).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e\u003cb\u003eGeometric parameters of CeCSc\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e \u003cb\u003ein C\u003c/b\u003e\u003csub\u003e\u003cb\u003e80\u003c/b\u003e\u003c/sub\u003e \u003cb\u003eand in aromatic ligand coordination\u003c/b\u003e\u003csup\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSpecies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003eBond Length (\u0026Aring;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDihedral Angle (\u0026deg;)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eL\u003csup\u003e\u003cem\u003eb\u003c/em\u003e\u003c/sup\u003e\u0026minus;Ce\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eCe\u0026thinsp;\u0026equiv;\u0026thinsp;C\u003c/b\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u0026thinsp;\u0026minus;\u0026thinsp;Sc1/ C\u0026thinsp;\u0026minus;\u0026thinsp;Sc2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSc1\u0026thinsp;\u0026minus;\u0026thinsp;L/ Sc2\u0026thinsp;\u0026minus;\u0026thinsp;L\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSc1\u0026thinsp;\u0026minus;\u0026thinsp;C\u0026minus;Sc2\u0026thinsp;\u0026minus;\u0026thinsp;Ce\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e (Expt.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e2.551\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e1.978\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.010/2.035\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003e2.407/2.433\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e172.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e (Cal.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e2.590\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e2.049\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.015/2.016\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003e2.412/20418\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e176.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e(C\u003csub\u003e5\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003e)CeC[Sc(C\u003csub\u003e8\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e)]\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (Cal.)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e2.754\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e1.968\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.170/2.128\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003e2.378/2.397\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e173.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003e \u003cem\u003ea\u003c/em\u003e \u003c/sup\u003eBond lengths (in \u0026Aring;, average values are italic) and bond angles (in \u003csup\u003eo\u003c/sup\u003e) are from X-ray diffraction of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e\u0026middot;[Ni-OEP] co-crystals and the computed data are from B3LYP/6-31G*+SDD calculation. \u003csup\u003e\u003cem\u003eb\u003c/em\u003e\u003c/sup\u003eIn CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e, L for Ce is the nearest benzene portion, for two Sc are the the nearest eight carbon atoms, including a benzene ring part and the two atoms next to it. In (C\u003csub\u003e5\u003c/sub\u003eH\u003csub\u003e5\u003c/sub\u003e)CeC[Sc(C\u003csub\u003e8\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e)]\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, L are the nearest carbon rings to the corresponding metal.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAtomic orbitals (AOs) contributions of NLMOs in CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e at B3LYP/6-31G*+SDD.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBond\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"6\" nameend=\"c7\" namest=\"c2\"\u003e \u003cp\u003eAOs% contributions\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCe\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4f/5d/6s\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2s/2p\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSc\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3d/4s\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCe\u0026ndash;C σ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e57/39/3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e29/71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e96/1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCe\u0026ndash;Cπ\u003csub\u003e||\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e40/60/0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0/100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e93/6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCe\u0026ndash;C π\u003csub\u003e\u0026perp;\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e42/58/0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0/100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e97/0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSc\u0026ndash;C\u0026ndash;Sc σ\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e13/61/25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e72/28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e64/35\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eMaterials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eReagents used for synthesis were: Sc\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (99.9%, J\u0026amp;K Scientific); CeO\u003csub\u003e2\u003c/sub\u003e (99.9%, Aladdin); Graphite rod (99.9%, Shanghai Fengyi Carbon Corporation); Graphite powder (99.9%, Shanghai Fengyi Carbon Corporation); Carbon disulfide (HPLC \u0026ge;99.9%, Aladdin); Acetone (AR, Chinasun Specialty Products Co., Ltd.); Toluene (AR, Aladdin); \u003cem\u003en\u003c/em\u003e-Hexane (HPLC \u0026ge;99.9%, Rhawn); Methanol (HPLC \u0026ge;99.9%, Meryer); Benzene (99.8%, Sigma-Aldrich); He (99.999%, purchased from Jinhong Gas).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSynthesis and Isolation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e was synthesized by a modified Krätschmer-Huffman direct-current arc-discharge method. The graphite rods, packed with CeO\u003csub\u003e2\u003c/sub\u003e, Sc\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and graphite power (molar ratio of Ce/Sc/C = 1: 1: 24), were vaporized in a VDK250 arcing reactor (Beijing Technol Science Co., Ltd., China) under a 110 A direct current and 200 Torr He atmosphere. The targeted Ce-based metallofullerenes were extracted from the carbon soot by CS\u003csub\u003e2\u003c/sub\u003e for 24 hours. This solution was then filtered through a sand core suction filtration device and the obtained filtrate was spin-dried with a rotary evaporator, then dissolved in toluene to obtain a crude extract. The crude extract was isolated using an LC-9230II NEXT recycling preparative HPLC machine (Japan Analytical Industry Co., Ltd., Japan). Four types of Cosmosil columns (Nacalai Tesque Inc., Japan), including a preparative Buckyprep-M (20 \u0026times; 250 mm), a semi-preparative Buckprep (10 \u0026times; 250 mm), a semi-preparative 5PYE (10 \u0026times; 250 mm), and a semi-preparative 5PBB (10 \u0026times; 250 mm), were utilized in the HPLC procedures.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMass spectroscopy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe positive-ion mode matrix-assisted laser desorption/ionization time-of-flight (Bruker, Germany) was employed for the mass characterization.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eVisible-near-infrared (Vis-NIR) absorption spectroscopy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe UV\u0026minus;vis\u0026minus;NIR spectrum of the purified CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e was measured in CS2 solution with a Cary 5000 UV\u0026minus;vis\u0026minus;NIR spectrophotometer (Agilent, U.S.).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFourier transform infrared (FTIR) absorption spectroscopy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Micro Fourier transform infrared spectra were measured at room temperature by a Vertex 70 + Hyperon 1000 spectrometer (Bruker, Germany) with a resolution of 4 cm\u003csup\u003e\u0026minus;1\u003c/sup\u003e and signal to noise ratio (7000:1).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSingle-crystal X-ray diffraction (SC-XRD)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe black block co-crystals of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e\u0026middot;[Ni\u003csup\u003eII\u003c/sup\u003e-OEP]\u003csup\u003e55\u003c/sup\u003e (OEP = 2, 3, 7, 8, 12, 13, 17, 18-octaethylporphyrin dianion) was obtained by slow diffusion of a benzene solution of Ni\u003csup\u003eII\u003c/sup\u003e-OEP into a carbon disulfide solution of the metallofullerene sample. Single-crystal X-ray data of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e was collected at 120 K on a diffractometer (APEX II; Bruker D8 Venture) equipped with a CCD collector. The multi-scan method was used for absorption correction. The refinement of the crystallographic structures was performed in the Olex2 software\u003csup\u003e56\u003c/sup\u003e packed with SHELXL-2018\u003csup\u003e57\u003c/sup\u003e by the full-matrix least-squares method.\u003c/p\u003e\n\u003cp\u003e\u003ca name=\"_Toc127029625\"\u003e\u003c/a\u003e\u003ca name=\"_Toc127114511\"\u003e\u003c/a\u003e\u003cstrong\u003eQuantum-chemical and Computational Methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe calculations were done with ADF code in AMS22\u003csup\u003e58\u003c/sup\u003e. The PBE functional\u003csup\u003e41\u003c/sup\u003e was used along with the zero-order regular approximation (ZORA)\u003csup\u003e59\u0026ndash;61\u003c/sup\u003e to account for the scalar relativistic (SR) effect. The spin-orbit (SO) coupling effect was not taken into account because it is expected not to have significant influence for these systems. Slater-type orbital (STO) basis sets\u003csup\u003e62\u003c/sup\u003e with small frozen core were utilized. Double-\u0026zeta; polarized (DZP) basis sets were used for carbon atoms on the fullerene cage, while triple-\u0026zeta; plus two polarization functions (TZ2P) were used for other atoms. Additionally, the same methods and non-frozen core TZ2P basis set were employed to compute the overlap integrals between Ce or Sc atoms and C atoms at experimental bond lengths. The singlet, triplet, quintet, and septet states of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e were optimized separately.\u003c/p\u003e\n\u003cp\u003eFurther optimizations of the singlet and triplet states were also performed using the B3LYP\u003csup\u003e42\u003c/sup\u003e functional implemented in Gaussian16\u003csup\u003e63\u003c/sup\u003e and the PBE0\u003csup\u003e43\u003c/sup\u003e functional in ORCA 5.0\u003csup\u003e64\u003c/sup\u003e for comparison. For Gaussian16 calculations, 6-31G* basis sets\u003csup\u003e65\u003c/sup\u003e were applied for carbons, whereas the SDD pseudopotential and corresponding basis sets, namely ECP28MWB\u003csup\u003e66,67\u003c/sup\u003e and ECP10MDF\u003csup\u003e68,69\u003c/sup\u003e, were used for Sc and Ce, respectively. In ORCA calculations, the def2-SVP basis set\u003csup\u003e70\u003c/sup\u003e was applied to carbons on the fullerene, def2-TZ2P-PP\u003csup\u003e66,71\u003c/sup\u003e for Ce, and def2-TZVP for other elements. And orbital transmission via the MOKIT program\u003csup\u003e72\u003c/sup\u003e was employed to expedite convergence. Despite using various methods like spin-flip in AMS, the fragment guess in Gaussian, and the broken symmetry in ORCA, the calculated spin-polarized singlet state always converged to a closed shell singlet, indicating that close-shell singlet is indeed the likely ground state. In addition, the calculated spin density of the triplet state CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e is mainly located on the Ce atom and the fullerene cage, while the experimental electron paramagnetic resonance (EPR) spectra did not detect any free radical signals on the fullerene cage, further confirming the stability of the singlet state.\u003c/p\u003e\n\u003cp\u003eThe infrared spectrum obtained via ORCA was scaled using a correction factor of 0.9572, derived from the experimental frequency of CO at 2143 cm\u003csup\u003e\u0026minus;1\u003c/sup\u003e. AMS program was used to provide Mayer bond orders\u003csup\u003e73\u003c/sup\u003e, Gopinathan-Jug (G-J) bond orders\u003csup\u003e74\u003c/sup\u003e, and Nalewajski-Mrozek #3 (N-M(3)) bond orders\u003csup\u003e48\u003c/sup\u003e. Natural Localized Molecular Orbital (NLMO) analysis\u003csup\u003e44\u003c/sup\u003e, Natural Resonance Theory (NRT) analysis\u003csup\u003e49,75\u003c/sup\u003e and Wiberg Bond Index (WBI) analysis\u003csup\u003e76\u003c/sup\u003e were performed using the NBO 6.0 program\u003csup\u003e77\u003c/sup\u003e. Additionally, Adaptive Natural Density Portioning (AdNDP),analysis\u003csup\u003e46\u003c/sup\u003e and \u0026lsquo;quantum theory of atoms-in-molecules\u0026rsquo; (QTAIM) analysis\u003csup\u003e50\u003c/sup\u003e were conducted using the Multiwfn program\u003csup\u003e78\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eReferences\u003c/h3\u003e\n\u003col start=\"55\"\u003e\n\u003cli\u003eOlmstead, M. M. \u003cem\u003eet al.\u003c/em\u003e Interaction of Curved and Flat Molecular Surfaces. 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Application of the pople-santry-segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane. \u003cem\u003eTetrahedron\u003c/em\u003e \u003cstrong\u003e24\u003c/strong\u003e, 1083\u0026ndash;1096 (1968).\u003c/li\u003e\n\u003cli\u003eGlendening, E. D., Landis, C. R. \u0026amp; Weinhold, F. NBO 6.0: Natural bond orbital analysis program. \u003cem\u003eJ. Comput. Chem.\u003c/em\u003e \u003cstrong\u003e34\u003c/strong\u003e, 1429\u0026ndash;1437 (2013).\u003c/li\u003e\n\u003cli\u003eLu, T. \u0026amp; Chen, F. Multiwfn: A multifunctional wavefunction analyzer. \u003cem\u003eJ. Comput. Chem.\u003c/em\u003e \u003cstrong\u003e33\u003c/strong\u003e, 580\u0026ndash;592 (2012).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5351349/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5351349/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMetal\u0026ndash;ligand multiple bonds are intriguing in coordination and organometallic chemistry. However, lanthanide\u0026ndash;carbon multiple bonds are extremely difficult to form. Despite decades of effort, isolable complexes containing lanthanide\u0026ndash;carbon double bonds with terminal methyl carbene (=\u0026thinsp;CH\u003csub\u003e2\u003c/sub\u003e) and lanthanide\u0026ndash;carbon triple bonds have never been found. Here, we report the successful synthesis of an unprecedented lanthanide\u0026ndash;carbon triple-bonded compound with cerium-carbide [Ce\u0026equiv;C\u0026thinsp;\u0026minus;\u0026thinsp;Sc\u003csub\u003e2\u003c/sub\u003e] cluster encapsulated inside a C\u003csub\u003e80\u003c/sub\u003e fullerene cage. The molecular structure of CeCSc\u003csub\u003e2\u003c/sub\u003e@C\u003csub\u003e80\u003c/sub\u003e and the nature of the Ce\u0026equiv;C triple bond are characterized through X-ray crystallography, spectroscopic analyses, and quantum chemical study, revealing a very short Ce\u0026equiv;C distance of 1.978(8) \u0026Aring;. Chemical bonding analysis suggests that the formation of the Ce\u0026equiv;C bond primarily arises from the stronger bonding affinity between carbon and cerium compared to scandium inside the encapsulated cluster. The fullerene cage plays a crucial role in stabilizing and protecting this trimetallic carbide cluster with a Ce\u0026equiv;C triple bond.\u003c/p\u003e","manuscriptTitle":"Endeavoring the First Lanthanide–Carbon Triple-Bond in Fullerene Cage","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-13 06:14:13","doi":"10.21203/rs.3.rs-5351349/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-chemistry","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nchem","sideBox":"Learn more about [Nature Chemistry](http://www.nature.com/nchem/)","snPcode":"","submissionUrl":"","title":"Nature Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"59d0605b-879e-4c9f-9c3f-804f1b7fedc5","owner":[],"postedDate":"November 13th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":40118482,"name":"Physical sciences/Chemistry/Inorganic chemistry/Chemical bonding"},{"id":40118483,"name":"Physical sciences/Chemistry/Coordination chemistry"}],"tags":[],"updatedAt":"2025-06-20T07:07:22+00:00","versionOfRecord":{"articleIdentity":"rs-5351349","link":"https://doi.org/10.1038/s41557-025-01856-2","journal":{"identity":"nature-chemistry","isVorOnly":false,"title":"Nature Chemistry"},"publishedOn":"2025-06-19 04:00:00","publishedOnDateReadable":"June 19th, 2025"},"versionCreatedAt":"2024-11-13 06:14:13","video":"","vorDoi":"10.1038/s41557-025-01856-2","vorDoiUrl":"https://doi.org/10.1038/s41557-025-01856-2","workflowStages":[]},"version":"v1","identity":"rs-5351349","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5351349","identity":"rs-5351349","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}