Perfect planar polyynic cyclo[n]carbon complexes [Cs©C18]+ and [Na©C14]+ with alkaline-metal centers exhibiting record coordination numbers and transition metal behaviors

Perfect planar polyynic cyclo[n]carbon complexes [Cs©C18]+ and [Na©C14]+ with alkaline-metal centers exhibiting record coordination numbers and transition metal behaviors | 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 Perfect planar polyynic cyclo[n]carbon complexes [Cs©C 18 ] + and [Na©C 14 ] + with alkaline-metal centers exhibiting record coordination numbers and transition metal behaviors Min Zhang, Rui-Nan Yuan, Yan-Bo Wu, Qiang Chen, Zhihong Wei, Si-Dian Li This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2614379/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Searching for the maximum coordination number (CN) in planar species with novel bonding patterns has fascinated chemists for many years. Using the experimentally observed cyclo[18]carbon D 9 h C 18 and theoretically predicted cyclo[14]carbon D 7 h C 14 as effective ligands and based on extensive first-principles theory calculations, we predict herein their perfect planar alkaline-metal-doped complexes D 9 h Cs©C 18 + ( 1 ) and D 7 h Na©C 14 + ( 4 ) which, as the global minima of the systems with an alkaline metal atom located exactly at the center, possess the record coordination numbers of CN = 18 and 14 in planar species, respectively. More interestingly, detailed energy decomposition and adaptive natural density partitioning bonding analyses indicate that the hypercoordinate alkaline-metal centers in these σ + π dually aromatic complexes exhibit obvious transition metal behaviors, with effective in-plane (π-6 s )σ, (π-7 p )σ, and (π-5 d )σ coordination bonds formed in Cs©C 18 + ( 1 ) and (π-3 s )σ, (π-3 p )σ, and (π-3 d )σ coordination interactions fabricated in Na©C 14 + ( 4 ) to dominate the overall attractive interactions between the metal center and its cyclo[n]carbon ligand. Similar dually aromatic alkaline-metal-centered planar C s Cs©C 17 B ( 2 ), C 2 v Cs©C 17 - ( 3 ), C 2 v Na©C 13 B ( 5 ), and C 2 v Na©C 13 - ( 6 ) have also been obtained with CN = 18, 17, 14, and 13, respectively. Physical sciences/Chemistry/Coordination chemistry Physical sciences/Chemistry/Inorganic chemistry Physical sciences/Chemistry/Materials chemistry Physical sciences/Chemistry/Theoretical chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The successive discoveries of fullerenes in 1985, 1 carbon nanotubes in 1991, 2 and graphene in 2004 3 which all consist exclusively of 3-coordinate carbon atoms have sparked a new field of synthetic carbon allotropes in chemistry. The recent characterization of polyynic cyclo[ 18 ]carbon D 9 h C 18 with obvious bond length alternations (BLA) in 2019 by high-resolution atomic force microscopy marks the onset of an alternative family of molecular carbon allotropes consisting solely of 2-coordinate carbon atoms in the cyclo[n]carbon ring series. 4 Previous gas-phase experiments indicated that cyclo[n]carbon rings as primary precursors may coalescence for form fullerenes and carbon nanotubes. 5,6 Electronic spectroscopic measurements showed that both C 18 and C 14 possess monocyclic geometries, though these studies did not reveal whether they have cumulenic or polyynic structures. 7,8 High level quantum Monte Carlo simulation and coupled cluster methods with single and double excitations (CCSD) investigations indicated that both polyynic D 9 h C 18 and D 7 h C 14 are the ground states of the systems due to second-order Jahn-Taller effects, with their cumulenic counterparts with no BLA always behaving as transition states. 9,10 Such perfect polyynic cyclo[n]carbon species and their in-plane and out-of-plane dual π-aromaticity have aroused considerable interests among chemists and presented viable possibilities to form planar metal-doped cyclo[n]carbon complexes with high coordination numbers (CN) and novel bonding patterns. A recent theoretical investigation 11 suggested that the Li-doped C 18 complex may serve as a potential optical switch which transforms between two stable C s configurations with Li inside (Li@C 18 in ) and outside the carbon ring (Li@C 18 out ). However, in the ground state (Li@C 18 in ) of such an alkaline-metal-doped cyclo[ 18 ]carbon complex, the Li atom with the coordination number of CN = 5 appears to be severely off-centered due to the size mismatch between Li and its monocyclic C 18 ligand. Similar situation happens in the recently proposed metal-doped M@C 16 complexes (M = Ca, Sc, Ti, V, Ce, U) in which the off-centered alkaline-earth, lanthanide, or actinide metal atoms have the coordination numbers between CN = 4 ~ 6, 12 again due to size effect. A recent first-principles theory investigation by our group indicated that, in the experimentally observed La©C 13 + , the La center with the large atomic radius of r La = 1.83 Å 13 matches the C 13 ligand perfectly both geometrically and electronically to form the perfect planar La-centered D 13 h La©C 13 + which has the highest coordination number of CN = 13 in planar species reported to date, demonstrating the unique coordinating capability of cyclo[n]carbon rings as effective ligands to metal centers in chemistry. 14 However, it still remains unknown to date in both experiments and theory whether or not metal-centered hypercoordinate planar cyclo[n]carbon complexes with CN > 13 can be achieved in chemistry. To achieve higher CNs in metal-centered cyclo[n]carbon complexes with CN = n > 13, it requires in chemical intuition that the metal centers have atomic radii greater than that of La. Searching for the maximum coordination number in planar species is more than a curiosity, it is to push the limits and ultimately to understand the essential concepts in chemistry. 14,15 To successfully design a metal-centered hypercoordinate planar complex, the metal center and its ligand must match both geometrically and electronically, i.e., they must have the right geometrical sizes and electronic configurations. The prototypical electron-deficient planar cyclo[n]boron rings have proven to be effective ligands to coordinate transition metal centers. Perfect σ + π dually aromatic wheel-like D 8 h Co©B 8 − , D 9 h Ru©B 9 − , D 9 h Rh©B 9 − , D 9 h Ir©B 9 − , D 10 h Ta©B 10 − , and D 10 h Nb©B 10 − with CN = 8, 9, 9, 9, 10, and 10 have been observed in gas phases in recent joint photoelectron spectroscopy and first-principles theory investigations. 15–20 These results present the possibility to form metal-centered hypercoordinate planar complexes using C n B m binary monocyclic rings as effective ligands, as indicated in the cases of the previously reported C 2 v Y©B 6 C 6 + and C 2 v Sc©B 5 C 6 . 14 Alkaline-earth metal centers in their perfect body-centered cubic carbonyl complexes O h M(CO) 8 + (M = Ca, Sr, or Ba) in low-temperature neon matrixes have been confirmed to be honorary transition metals with effective M–(CO) 8 (π) coordination interactions. 21 Similar M( d π )–(CO) 8 (π) coordination bonds were predicted to exist in O h M(CO) 8 − complexes (M = K, Rb) in which the alkaline metal centers K and Rb exhibit transition metal behaviours. 22 Given the fact that alkaline metals possess the largest atomic radii in the periodical table 23 and have the potential to form complexes with transition metal behaviors, it is possible to form alkaline-metal-doped cyclo[n]carbon complexes ( n ≥ 14) or their boron-substituted derivatives with CN ≥ 14 if the alkaline metal center and its ligand are chosen properly to match both geometrically and electronically. Keeping the inspirations in mind, using the experimentally observed perfect planar ring-like D 9 h C 18 and theoretically predicted D 7 h C 14 as ligands and based on extensive global minimum searches augmented with first-principles theory calculations, we predict in this work the perfect planar alkaline-metal-centered D 9 h Cs©C 18 + ( 1 ) and D 7 h Na©C 14 + ( 4 ) which have the record coordination numbers of CN = 18 and 14 in planar species, respectively. Cs and Na with the atomic radii of r Cs = 2.65 Å and r Na = 1.86 Å 13 prove to match the D 9 h C 18 and D 7 h C 14 ligands perfectly both geometrically and electronically, respectively. Effective in-plane (π- s )σ, (π- p )σ, and (π- d )σ coordination bonds are formed to dominate the attractive interactions in these novel complexes in which the alkaline-metal centers exhibit transition metal behaviors. The iso-chemical shielding surfaces and out-of-plane π and in-plane σ ring current maps of the concerned species are computationally simulated to evidence their σ + π dual aromaticity. Computational Procedures Extensive global-minimum (GM) searches were performed on Cs©C 18 + , Na©C 14 + , Cs©C 17 B, Cs©C 17 − , Na©C 13 B, and Na©C 13 − using the TGmin2 code 24 at DFT level based on the basin-hopping algorithm. 25 Over 1000 stationary points were explored for each species at PBE/DZVP level employing the CP2K program. 26,27 The low-lying isomers were then fully optimized at both M06-2X and ωB97XD levels 28,29 with vibrational frequencies checked, with the aug-cc-pvtz basis set for C, B, Na, and K and Stuttgart relativistic small-core pseudopotentials 30,31 for Rb, Cs, and Fr, using the Gaussian16 program. 32 The fact that M06-2X produces essentially the same polyynic D 9h C 18 and D 7h C 14 structures (Fig. S1 ) as that obtained at the more accurate QMC and CCSD levels 9,10 evidences the reliability of the optimized geometries depicted in Figure.1. Natural bonding orbital (NBO) analyses were implemented using NBO 7.0 program. 33 The energy decomposition analyses (EDA) together with the natural orbitals for chemical valence (NOCV) calculations, denoted as EDA-NOCV, 34,36 were carried out with the ADF program package 37 at M06-2X/TZ2P 38 level where scalar relativistic effects were considered for Cs using the zero order regular approximation (ZORA). 39 The frozen core approximation was not employed in EDA-NOCV computations. The overall interaction energy (Δ E int ) between two fragments is divided into three main terms: the electrostatic interaction energy (Δ E elstat ), Pauli repulsion (Δ E Pauli ), and orbital interaction energy (Δ E orb ) in Eq. (1): Δ E int = Δ E elstat + Δ E Pauli + Δ E orb . (1) Detailed bonding analyses on D 9 h Cs©C 18 + ( 1 ), D 7 h Na©C 14 + ( 4 ), and C s Cs©C 17 B (2) were implemented using the adaptive natural density partitioning (AdNDP 2.0) approach 40,41 at the M06-2X/6-31G level, with the isosurface maps of the orbitals visualized using the Visual Molecular Dynamics (VMD) software. 42 The iso-chemical shielding surfaces (ICSSs) 43,44 and isosurfaces of localized orbital locators (LOL) 45 were obtained with Multiwfn 3.8 code. 46 The anisotropy of the current-induced density (ACID) analyses were realized by the ACID code, 47 with the maps finally generated by POV-Ray render. 48 Results Structures and Stability The optimized GM structures of D 9 h Cs©C 18 + ( 1 ), C s Cs©C 17 B ( 2 ), C 2 v Cs©C 17 − ( 3 ), D 7 h Na©C 14 + ( 4 ), C 2 v Na©C 13 B ( 5 ), and C 2 v Na©C 13 − ( 6 ) are collectively plotted in Fig. 1 , with more alternative isomers summarized in Figures S3-S8. Figure S2 depicts the optimized GM structures of (a) the alkaline-metal-doped cyclo[ 18 ]carbon complexes M©C 18 + with M = Li, Na, K, Rb, Cs, and Fr and (b) alkaline-metal-doped cyclo[ 14 ]carbon derivatives M©C 14 + with M = Li, Na, and K at M06-2X. It is noticed that the alkaline metal atoms in the GMs are all located inside the cyclo[n]carbon rings, with the alkaline metal atoms severely off-centered in C 2 v Li©C 18 + , C 2 v Na©C 18 + , C 2 v K©C 18 + , and C 2 v Li©C 14 + and slightly off-centered in C s Rb©C 18 + and C s Fr©C 18 + . The K atom in C 7 v K©C 14 + lies about 1.14 Å above the ligand plane along the C 7 molecular axis due to its large atomic radius (r K = 2.32 Å) which appears to be too big to be hosted inside the C 14 ring. Encouragingly, Cs proves to have the right atomic radius of r Cs = 2.65 Å to be coordinated exactly at the center of the D 9 h C 18 ligand in D 9 h Cs©C 18 + ( 1 ) to achieve the highest coordination number of CN = 18 reported to date. As the well-defined GM of the complex (Fig. S3), Cs©C 18 + ( 1 ) exhibits the alternating bond lengths of r C–C = 1.343 Å and r C≡C = 1.224 Å at M06-2X which are well inherited from its parent ligand D 9 h C 18 ligand with r C–C = 1.343 Å and r C≡C = 1.223 Å at the same theoretical level (Fig. S1 ). The large calculated HOMO-LUMO gap of Δ E gap = 5.38 eV at M06-2X well supports its high chemical stability. The second isomer C 2 v Cs©C 18 + with a Cs + located outside the C 18 ring and the seventh isomer C 2 v Cs©C 18 + with a Cs + inserted into the C 18 ring appear to be 0.38 eV and 4.79 eV less stable than D 9 h GM at M06-2X, respectively (Fig. S3). The slightly off-centered planar C s Rb©C 18 + and C s Fr©C 18 + also possess the coordination numbers of CN = 18 (Fig. S2). Both the planar neutral C s Cs©C 17 B ( 2 ) which is isoelectronic with Cs©C 18 + ( 1 ) with obviously bond-length alternations and C 2 v Cs©C 17 - (3) with roughly averaged bond lengths are the well-defined GMs of the systems with CN = 18 and 17, respectively (Fig. S4 and Fig. S5). However, the severely off-centered C 2 v Li©C 18 + , C 2 v Na©C 18 + , and C 2 v K©C 18 + with obvious smaller alkaline metal centers Li, Na, and K appear to have much smaller coordination numbers with CN = 4 ~ 6 (Fig. S2). Similarly, Na appears to have the right atomic radius (r Na = 1.86 Å) to be hosted exactly at the center of the D 7 h C 14 ligand to form the perfect planar polyynic D 7 h Na©C 14 + ( 4 ) (Fig. S6) with CN = 14. The second lowest-lying isomer C s Na©C 14 + with Na + outside the C 14 ring lies only 0.23 eV higher than Na©C 14 + ( 4 ) (Fig. S6). The two close-lying lowest-lying isomers of Cs©C 18 + and Na©C 14 + discussed above (Fig. S3 and Fig. S6) may transform between each other with low energy barriers under certain conditions. Na©C 14 + ( 4 ) as the GM of the system has the alternating bond lengths of r C–C = 1.326 Å and r C≡C = 1.240 Å at M06-2X well comparable with the corresponding values of r C–C = 1.324 Å and r C≡C = 1.237 Å calculated for D 7 h C 14 at the same theoretical level (Fig. S1 ), while Li with the atomic radius of r Li = 1.52 Å proves to be too small and K with r K = 2.32 Å appears to be too big to be hosted at the ring center of the C 14 ligand, they form severely off-centered and off-planed structures, respectively (Fig. S2). With the HOMO-LUMO gap of Δ E gap = 5.87 eV, Na©C 14 + (4) is expected to have high chemical stability. The slightly off-centered planar C 2 v Na©C 13 B ( 5 ) with CN = 14 and vibrationally averaged C 2 v Na©C 13 - ( 6 ) with CN = 13 with roughly averaged bond lengths also appear to be the well-defined GMs of the systems (Fig. S7 and Fig. S8). As expected, the high-symmetry Cs©C 18 + ( 1 ) and Na©C 14 + ( 4 ) exhibit highly characteristic calculated vibrational spectroscopic features as shown in Fig. S9, with the former possessing well characterized IR peaks at 513 and 2202 cm − 1 and Raman active vibrations at 1792 and 2293 cm − 1 , respectively, while the latter having two well separated IR peaks at 545 and 2160 cm − 1 and one dominant Raman feature at 1252 cm − 1 . Such well-defined spectral features can help facilitate future experimental characterizations of these species. EDA-NOCV Bonding Scheme Analyses To shed insights into the bonding nature of D 9 h Cs©C 18 + ( 1 ) and D 7 h Na©C 14 + ( 4 ), detailed EDA-NOCV analyses were carried out at M06-2X/TZ2P. The D 3h subgroup was applied to D 9h Cs©C 18 + ( 1 ) because the highest point group supported by ADF program is D 8h . It was found that Cs + and C 18 as the most possible reacting fragments give the most favorite interaction energy of Δ E int = -15.22 kcal/mol for Cs©C 18 + ( 1 ) in different fragmental schemes (Table S1 ). They are thus chosen as interacting species to demonstrate the bonding scheme of Cs©C 18 + ( 1 ) in Fig. 2 (a). Similarly, Na + and C 14 as reacting fragments with Δ E int = -24.44 kcal/mol are chosen for Na©C 14 + ( 4 ) in Fig. 2 (b). The bonding molecular orbitals (MOs) 15a 1 ’, 19e 1 ’ and 20e 1 ’ of D 3 h Cs©C 18 + representing covalent bonding MOs between Cs + and C 18 are connected with the corresponding fragmental orbitals by bold dashed lines in Fig. 2 (a), with the orbital compositions tabulated in Table S2. The non-degenerate 15a 1 ’ mainly originates from the occupied 8a 1 ’ of C 18 with in-plane π characteristics (abbreviated as π in ) and vacant 6 s of Cs + by (π-6 s )σ coordination interactions, the doubly degenerate 19e 1 ’ is composed of the occupied in-plane 13e 1 ’ (π in ) of C 18 with one nodal plane and vacant 7 p x and 7 p y of Cs + by (π-7 p )σ coordination, while the doubly degenerate 20e 1 ’ is composed of the occupied 14e 1 ’ of C 18 with π in characteristics with two nodal planes and vacant \({5d}_{xy}\) and \({5d}_{{x}^{2}-{y}^{2}}\) of Cs + by (π-5 d )σ coordination. As detailed in Table 1 , EDA analyses demonstrate that the overall interaction energy of Δ E int = -15.22 kcal/mol between the Cs + and C 18 in Cs©C 18 + consists of the Pauli repulsion Δ E Pauli = 1.89 kcal/mol, Coulombic attraction Δ E elstat = -3.16 kcal/mol, and orbital interaction Δ E orb = -13.95 kcal/mol, with covalent orbital interaction making a dominating contribution of 81.5% to the overall attraction interaction (-17.11 kcal/mol), while electrostatic attraction contributing only 18.5%. The decompositions of the orbital interactions Δ E orb into pairwise contributions between occupied and vacant MOs of the fragments provide quantitative insight into the charge flow. The strongest orbital interaction Δ E orb(1) (20e 1 ’, 27.5%) arises mainly from [C 18 (π in )] → [Cs + (5 d )] where C 18 serves as a π in -donor to coordinate the \({5d}_{xy}\) and \({5d}_{{x}^{2}-{y}^{2}}\) orbitals of the Cs + as σ-acceptors. The orbital interaction Δ E orb(2) (19e 1 ’, 16.8%) originates from [C 18 (π in )] → [Cs + (7 p )] where the 7 p x and 7 p y orbitals of the Cs + serve as σ-acceptors. The orbital interaction Δ E orb(3) (15a 1 ’, 14.1%) originates from [C 18 (π in )] → [Cs + (6 s )] where the 6 s orbital of the Cs + is a σ-acceptor. Fig. S10 shows the corresponding deformation densities Δ ρ associated with the pairwise interactions Δ E orb(1) , Δ E orb(2) and Δ E orb(3) in Cs©C 18 + , further indicating that C 18 serves as a π in -donor while Cs + is a σ-acceptor in the complex. Detailed EDA-NOCV calculations for D 7 h Na©C 14 + ( 4 ) gives a similar trend as shown in Fig. 2 (b) and Table 1 . The bonding MOs 6a 1 ’ , 6e 1 ’ and 5e 2 ’ representing covalent bonding interactions between the Na + and C 14 fragmental orbitals are connected by bold dashed lines with the corresponding fragmental orbitals, with the orbital compositions listed in Table S3. The 6a 1 ’ mainly originates from the occupied 4a 1 ’ of C 14 with π in characteristics and vacant 3 s of Na + by (π-3 s )σ coordination interactions, the doubly degenerate 6e 1 ’ is composed of occupied 5e 1 ’ of C 14 with π in characteristics and vacant 3 p x and 3 p y of Na + by (π-3p)σ coordination, while the 5e 2 ’ is composed of C 14 with π in characteristics and vacant \({3d}_{xy}\) and \({3d}_{{x}^{2}-{y}^{2}}\) of Na + by (π- 3d )σ coordination. EDA analyses (Table 1 ) indicate that overall attraction interaction is overwhelmingly dominated by covalent orbital contribution (94.0%), while electrostatic attraction makes only a marginal contribution (6.0%). The decompositions of Δ E orb into pairwise contributions between occupied and vacant MOs of the fragments reveals that the strongest orbital interaction Δ E orb(1) (24.9%) originates mainly from [C 14 (π in )] → [Na + (3 p )], the orbital interaction Δ E orb(2) (19.2%) arises mainly from [C 14 (π in )] → [Na + (3 s )], while the orbital interaction Δ E orb(3) (18.1%) originates from [C 14 (π in )] → [Na + (3 d ). The corresponding deformation densities Δ ρ associated with the pairwise interactions Δ E orb(1) , Δ E orb(2) and Δ E orb(3) in Na©C 14 + in Fig. S11 clearly indicate that C 14 serves as a π in -donor while Na + is a σ-acceptor. Table 1 EDA-NOCV results for Cs©C 18 + ( 1 ) and Na©C 14 + ( 4 ) at the M06-2X/TZ2P-ZORA level, taking C 18 with Cs + and C 14 with Na + as interacting fragments, respectively. Energy values are given in kcal/mol. Energy terms interaction Cs + + C 18 interaction Na + + C 14 Δ E int -15.22 -24.44 Δ E elstat a -3.16 (18.5%) -1.42 (6.0%) Δ E Pauli 1.89 3.45 Δ E orb a -13.95 (81.5%) -22.41 (94.0%) Δ E orb(1) b C 18 (π in ) donation→[Cs + (5 d )] -3.84 (27.5%) C 14 (π in ) donation→[Na + (3 p )] -5.58 (24.9%) Δ E orb(2) b C 18 (π in ) donation→[Cs + (7 p )] -2.34 (16.8%) C 14 (π in ) donation→[Na + (3 s )] -4.30 (19.2%) Δ E orb(3) b C 18 (π in ) donation→[Cs + (6 s )] -1.96 (14.1%) C 14 (π in ) donation→[Na + (3 d )] -4.06 (18.1%) Δ E orb(rest) b -5.81 (41.6%) -8.47 (37.8%) a The value in parentheses gives the percentage contribution to the total attractive interactions (Δ E elstat + Δ E orb ); b The value in parentheses gives the percentage contribution to the total orbital interaction (Δ E orb ) The EDA-NOCV results detailed above quantitatively indicate that the cyclo[n]carbon ligands serve as good π in -donors to stabilize alkaline metal centers in both Cs©C 18 + ( 1 ) and Na©C 14 + ( 4 ) by donating their π in valence electrons partially to the vacant s, p , and d orbitals of Cs + and Na + through effective in-plane (π- s )σ, (π- p )σ, and (π- d )σ coordination interactions. Localized orbital locator (LOL) is an effective space function in revealing the distributions of delocalized electrons on conjugated rings in molecules. We calculated in-plane LOL-σ, in-plane LOL-π in , and out-of-plane LOL-π out separately based on the corresponding in-plane σ MOs, in-plane π MOs, and out-of-plane π MOs of the systems, respectively. To better reflect spatial distributions of LOL-σ, LOL-π in , and LOL-π out in Cs©C 18 + ( 1 ) and Na©C 14 + ( 4 ), the color-filled maps of LOL-σ on the ring plane, LOL-π in on the ring plane, and LOL-π out 1 Å above the ring plane are plotted in Fig. 3 (a) and Fig. 3 (b) comparatively. By comparing the area colors on the maps, it can be clearly seen that both LOL-π in and LOL-π out exhibit heavy density distributions over the short C ≡ C bonds and light density distributions over the long C-C bonds, well supporting the alternating of triple and single bonds in different bond lengths in both polyynic Cs©C 18 + ( 1 ) and Na©C 14 + ( 4 ). AdNDP Bonding Pattern Analyses Detailed AdNDP analyses in Fig. 3 (c) and (d) unveil both the localized and delocalized bonds in D 9 h Cs©C 18 + ( 1 ) and D 7 h Na©C 14 + ( 4 ) more vividly. As expected, out of the 72 valence electrons in Cs©C 18 + ( 1 ), 36 electrons form 18 equivalent 2c-2e C-C peripheral in-plane σ bonds with the occupation numbers of ON = 2.00 |e|. The remaining 36 valence electrons are distributed in two types of chemical bonds, including 9 equivalent in-plane 3c-2e σ bonds on nine CsC 2 triangles with ON = 1.83 |e| and 9 equivalent out-of-plane 2c-2e C-C π bonds with ON = 1.83 |e|, respectively. Such a bonding pattern follows the 4 N + 2 aromatic rule for σ aromaticity with N σ = 4 and π aromaticity with N π = 4, respectively, making the planar complex σ + π dually aromatic in nature and rendering extra stability to the system, similar to the situation in the previously reported D 9 h C 18 . 11 Similarly, as shown in Fig. 3 (d), D 7 h Na©C 14 + ( 4 ) possesses 7 equivalent 2c-2e C-C periphery in-plane σ bonds, 7 equivalent in-plane 3c-2e σ bonds on seven NaC 2 triangles, and 7 equivalent out-of-plane 2c-2e C-C π bonds, again following the 4 N + 2 aromatic rule with N σ = N π = 3 for σ + π dual aromaticity. Similar bonding patterns exist in C s Cs©C 17 B ( 2 ) (Fig. S12). The dual aromaticities of both Cs©C 18 + ( 1 ) and Na©C 14 + ( 4 ) are also well supported by their delocalized in-plane σ MOs and delocalized out-of-plane π MOs shown in Fig. S13. The simulated ICSS isosurfaces of D 9 h Cs©C 18 + ( 1 ) and D 7 h Na©C 14 + ( 4 ) based on the ZZ components of the calculated nuclear-independent chemical shifts (NICS-ZZ) are presented as Fig. 4 (a), in comparison with that of the previously reported σ + π dually aromatic D 9 h C 18 and D 7 h C 14 . It can be clearly seen that, similar to D 9 h C 18 and D 7 h C 14 , both D 9 h Cs©C 18 + ( 1 ) and D 7 h Na©C 14 + ( 4 ) are aromatic in nature, with the spaces inside the cyclo[n]carbon rings and within ~ 1.0 Å above the ring planes belonging to chemical shielding areas with negative NICS-ZZ values (highlighted in yellow) and the blet-like regions around the cyclo[n]carbon rings in horizontal direction belonging to chemical deshielding areas with positive NICS-ZZ values (highlighted in green). The widely used ACID method can be employed to display graphically the ring currents induced by an external magnetic field in vertical directions perpendicular to the cyclo[n]carbon ring. Figure 4 (b) presents the calculated out-of-plane π and in-plane σ ring currents maps for both D 9 h Cs©C 18 + ( 1 ) and D 7 h Na©C 14 + ( 4 ), in comparison with the corresponding ring currents obtained for D 9 h C 18 and D 7 h C 14 at the same theoretical level, respectively. As clearly indicated in Fig. 4 (b), these alkaline-metal-centered polyynic complex monocations do possess intrinsic σ aromaticity and π aromaticity simultaneously, similar to their neutral parent ligands D 9 h C 18 and D 7 h C 14 in ring current distributions. Conclusions In summary, based on extensive first-principles theory calculations, we have predicted in this work a series of alkaline-metal-doped complexes Cs©C 18 + ( 1 ), Cs©C 17 B ( 2 ), Cs©C 17 − ( 3 ), Na©C 14 + ( 4 ), Na©C 13 B ( 5 ), and Na©C 13 − ( 6 ) which turn out to be GMs of the systems with the record coordination numbers of CN = 18 ~ 13 in planar species. These σ + π dually aromatic complexes possess effective in-plane (π- s )σ, (π- p )σ, and (π- d )σ coordination interactions which dominate the attractive interaction between the alkaline metal center as σ-acceptor and its cyclo[n]carbon ligand as in-plane π-donor, evidencing the transition metal behaviors of the alkaline metal centers in them. Similar to the situation in the recently observed alkaline-earth metal carbonyl species, 21 the perfect planar alkaline metal-centered polyynic cyclo[n]carbon complexes proposed in this work (n = 18, 14) with relatively low coordination energies may be produced in gas phases by laser ablation of alkaline-metal-carbon mixed binary targets and characterized by spectroscopic measurements at low temperatures to further push the boundary of coordination chemistry. Declarations Data availability The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. Competing interests There are no conflicts to declare. Acknowledgements The work was supported by the National Natural Science Foundation of China (21720102006, 21973057 and 22003034) and Natural Science Foundation of Shanxi Province of China (20210302124002). Author contributions S.D.Li, Z.H Wei, and Q, Chen conceived the project and finalized the manuscript. M.Z and R.N.Y did the calculations and prepared the first draft. Y.B.Wu helped analyze the data. All authors approved the final version. References Kroto, H. W., Heath, J. R., O’Brien, S. C., Curl, R. F. & Smalley, R. E. C 60 : Buckminsterfullerene. Nature 318, 162–163 (1985). Iijima S. Helical Microtubules of Graphitic Carbon. Nature 354, 56–68 (1991). Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grigorieva, I. V. & Firsov, A. A. Electric field effect in atomically thin carbon films. Science 306, 666–669 (2004). Kaiser, K., Scriven, L. M., Schulz, F., Gawel, P., Gross, L. & Anderson, H. 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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-2614379","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":178931311,"identity":"09250af1-a109-466f-a7d3-7aaa2a2d991c","order_by":0,"name":"Min Zhang","email":"","orcid":"","institution":"Shanxi University","correspondingAuthor":false,"prefix":"","firstName":"Min","middleName":"","lastName":"Zhang","suffix":""},{"id":178931313,"identity":"ea040721-f919-4779-95b9-dcb7c246b103","order_by":1,"name":"Rui-Nan Yuan","email":"","orcid":"","institution":"Shanxi 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03:44:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2614379/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2614379/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":33632616,"identity":"d1f84abe-358b-4846-a26c-9efcc5c0f929","added_by":"auto","created_at":"2023-03-01 16:03:45","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":125280,"visible":true,"origin":"","legend":"\u003cp\u003eOptimized structures of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (1), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e17\u003c/sub\u003eB (2), C\u003csub\u003e2\u003c/sub\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (3), \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (4), C\u003csub\u003e2\u003c/sub\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e13\u003c/sub\u003eB (5), and C\u003csub\u003e2\u003c/sub\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (6) at M06-2X level.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-2614379/v1/e2e7aaa0577afbd9f5c69e4a.png"},{"id":33633796,"identity":"d47c6c69-ea5a-465d-ace7-40dc77966005","added_by":"auto","created_at":"2023-03-01 16:11:46","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":146580,"visible":true,"origin":"","legend":"\u003cp\u003e(a) MO bonding scheme of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with the fragments of C\u003csub\u003e18\u003c/sub\u003e and Cs\u003csup\u003e+\u003c/sup\u003e as interacting species and (b) MO bonding scheme of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with C\u003csub\u003e14\u003c/sub\u003e and Na\u003csup\u003e+\u003c/sup\u003e as interacting species at M06-2X/TZ2P-ZORA level.\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-2614379/v1/a94a13047107cef4b7151924.png"},{"id":33633795,"identity":"7f7b3676-7d79-48ab-a18b-3e48d9ef7f40","added_by":"auto","created_at":"2023-03-01 16:11:45","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":327964,"visible":true,"origin":"","legend":"\u003cp\u003eColor-filled maps of the localized orbital locator isosurfaces of the in-plane σ\u003csub\u003ein\u003c/sub\u003e MOs, in-plane π\u003csub\u003ein\u003c/sub\u003e MOs, and out-of-plane π\u003csub\u003eout\u003c/sub\u003e MOs of (a)\u003cem\u003e \u003c/em\u003eCs©C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e1\u003c/strong\u003e) and (b) Na©C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e4\u003c/strong\u003e) and their corresponding AdNDP bonding patterns (c) and (d) with occupation numbers (ONs) indicated.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-2614379/v1/7004ff0afcfbfa1b7f205c51.png"},{"id":33632617,"identity":"c10cbdf3-3d9c-4377-b0af-adf04500caf1","added_by":"auto","created_at":"2023-03-01 16:03:46","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":475861,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Isosurface maps of calculated iso-chemical shielding surfaces (ICSSs) of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e1\u003c/strong\u003e), \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e, \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e4\u003c/strong\u003e), and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e. Yellow and green regions stand for chemical shielding and deshielding areas, respectively. (b) Calculated out-of-plane-π and in-plane-σ ring current maps of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e1\u003c/strong\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e4\u003c/strong\u003e), compared with the corresponding ring current maps of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and\u003cem\u003e D\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e \u003c/em\u003eC\u003csub\u003e14\u003c/sub\u003e, respectively. The external magnetic field is perpendicular to the ring plane. The red arrows represent directions and magnitudes of the ring currents at various positions on the ACID iso-surfaces.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-2614379/v1/37c8b409d01edaf57679c19c.png"},{"id":38326493,"identity":"7eca4961-da4b-4c2c-b1c8-53deb86975b0","added_by":"auto","created_at":"2023-06-10 08:59:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1242385,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2614379/v1/6e66eeb8-07db-4ec2-9471-9886328ed3fe.pdf"},{"id":33632620,"identity":"26330755-74f4-4846-b2d2-9f7070d28090","added_by":"auto","created_at":"2023-03-01 16:03:46","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1364181,"visible":true,"origin":"","legend":"","description":"","filename":"PaperonC18CsSci.Rep.2023222SI.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2614379/v1/4afec40dc35931b2456be79c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003ePerfect planar polyynic cyclo[n]carbon complexes [Cs©C\u003csub\u003e18\u003c/sub\u003e]\u003csup\u003e+\u003c/sup\u003e and [Na©C\u003csub\u003e14\u003c/sub\u003e]\u003csup\u003e+\u003c/sup\u003e with alkaline-metal centers exhibiting record coordination numbers and transition metal behaviors\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe successive discoveries of fullerenes in 1985,\u003csup\u003e1\u003c/sup\u003e carbon nanotubes in 1991,\u003csup\u003e2\u003c/sup\u003e and graphene in 2004\u003csup\u003e3\u003c/sup\u003e which all consist exclusively of 3-coordinate carbon atoms have sparked a new field of synthetic carbon allotropes in chemistry. The recent characterization of polyynic cyclo[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]carbon \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e with obvious bond length alternations (BLA) in 2019 by high-resolution atomic force microscopy marks the onset of an alternative family of molecular carbon allotropes consisting solely of 2-coordinate carbon atoms in the cyclo[n]carbon ring series.\u003csup\u003e4\u003c/sup\u003e Previous gas-phase experiments indicated that cyclo[n]carbon rings as primary precursors may coalescence for form fullerenes and carbon nanotubes.\u003csup\u003e5,6\u003c/sup\u003e Electronic spectroscopic measurements showed that both C\u003csub\u003e18\u003c/sub\u003e and C\u003csub\u003e14\u003c/sub\u003e possess monocyclic geometries, though these studies did not reveal whether they have cumulenic or polyynic structures.\u003csup\u003e7,8\u003c/sup\u003e High level quantum Monte Carlo simulation and coupled cluster methods with single and double excitations (CCSD) investigations indicated that both polyynic \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e are the ground states of the systems due to second-order Jahn-Taller effects, with their cumulenic counterparts with no BLA always behaving as transition states.\u003csup\u003e9,10\u003c/sup\u003e Such perfect polyynic cyclo[n]carbon species and their in-plane and out-of-plane dual π-aromaticity have aroused considerable interests among chemists and presented viable possibilities to form planar metal-doped cyclo[n]carbon complexes with high coordination numbers (CN) and novel bonding patterns. A recent theoretical investigation\u003csup\u003e11\u003c/sup\u003e suggested that the Li-doped C\u003csub\u003e18\u003c/sub\u003e complex may serve as a potential optical switch which transforms between two stable \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e configurations with Li inside (Li@C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003ein\u003c/sup\u003e) and outside the carbon ring (Li@C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003eout\u003c/sup\u003e). However, in the ground state (Li@C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003ein\u003c/sup\u003e) of such an alkaline-metal-doped cyclo[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]carbon complex, the Li atom with the coordination number of CN\u0026thinsp;=\u0026thinsp;5 appears to be severely off-centered due to the size mismatch between Li and its monocyclic C\u003csub\u003e18\u003c/sub\u003e ligand. Similar situation happens in the recently proposed metal-doped M@C\u003csub\u003e16\u003c/sub\u003e complexes (M\u0026thinsp;=\u0026thinsp;Ca, Sc, Ti, V, Ce, U) in which the off-centered alkaline-earth, lanthanide, or actinide metal atoms have the coordination numbers between CN\u0026thinsp;=\u0026thinsp;4\u0026thinsp;~\u0026thinsp;6,\u003csup\u003e12\u003c/sup\u003e again due to size effect. A recent first-principles theory investigation by our group indicated that, in the experimentally observed La\u0026copy;C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, the La center with the large atomic radius of r\u003csub\u003eLa\u003c/sub\u003e = 1.83 \u0026Aring; \u003csup\u003e13\u003c/sup\u003e matches the C\u003csub\u003e13\u003c/sub\u003e ligand perfectly both geometrically and electronically to form the perfect planar La-centered \u003cem\u003eD\u003c/em\u003e\u003csub\u003e13\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e La\u0026copy;C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e which has the highest coordination number of CN\u0026thinsp;=\u0026thinsp;13 in planar species reported to date, demonstrating the unique coordinating capability of cyclo[n]carbon rings as effective ligands to metal centers in chemistry.\u003csup\u003e14\u003c/sup\u003e However, it still remains unknown to date in both experiments and theory whether or not metal-centered hypercoordinate planar cyclo[n]carbon complexes with CN\u0026thinsp;\u0026gt;\u0026thinsp;13 can be achieved in chemistry. To achieve higher CNs in metal-centered cyclo[n]carbon complexes with CN\u0026thinsp;=\u0026thinsp;\u003cem\u003en\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;13, it requires in chemical intuition that the metal centers have atomic radii greater than that of La.\u003c/p\u003e \u003cp\u003eSearching for the maximum coordination number in planar species is more than a curiosity, it is to push the limits and ultimately to understand the essential concepts in chemistry.\u003csup\u003e14,15\u003c/sup\u003e To successfully design a metal-centered hypercoordinate planar complex, the metal center and its ligand must match both geometrically and electronically, i.e., they must have the right geometrical sizes and electronic configurations. The prototypical electron-deficient planar cyclo[n]boron rings have proven to be effective ligands to coordinate transition metal centers. Perfect σ\u0026thinsp;+\u0026thinsp;π dually aromatic wheel-like \u003cem\u003eD\u003c/em\u003e\u003csub\u003e8\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Co\u0026copy;B\u003csub\u003e8\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Ru\u0026copy;B\u003csub\u003e9\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Rh\u0026copy;B\u003csub\u003e9\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Ir\u0026copy;B\u003csub\u003e9\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, \u003cem\u003eD\u003c/em\u003e\u003csub\u003e10\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Ta\u0026copy;B\u003csub\u003e10\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e10\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Nb\u0026copy;B\u003csub\u003e10\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e with CN\u0026thinsp;=\u0026thinsp;8, 9, 9, 9, 10, and 10 have been observed in gas phases in recent joint photoelectron spectroscopy and first-principles theory investigations.\u003csup\u003e15\u0026ndash;20\u003c/sup\u003e These results present the possibility to form metal-centered hypercoordinate planar complexes using C\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003eB\u003csub\u003e\u003cem\u003em\u003c/em\u003e\u003c/sub\u003e binary monocyclic rings as effective ligands, as indicated in the cases of the previously reported C\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Y\u0026copy;B\u003csub\u003e6\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and C\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Sc\u0026copy;B\u003csub\u003e5\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e.\u003csup\u003e14\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAlkaline-earth metal centers in their perfect body-centered cubic carbonyl complexes \u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e M(CO)\u003csub\u003e8\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (M\u0026thinsp;=\u0026thinsp;Ca, Sr, or Ba) in low-temperature neon matrixes have been confirmed to be honorary transition metals with effective M\u0026ndash;(CO)\u003csub\u003e8\u003c/sub\u003e (π) coordination interactions.\u003csup\u003e21\u003c/sup\u003e Similar M(\u003cem\u003ed\u003c/em\u003e\u003csub\u003eπ\u003c/sub\u003e)\u0026ndash;(CO)\u003csub\u003e8\u003c/sub\u003e (π) coordination bonds were predicted to exist in \u003cem\u003eO\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e M(CO)\u003csub\u003e8\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e complexes (M\u0026thinsp;=\u0026thinsp;K, Rb) in which the alkaline metal centers K and Rb exhibit transition metal behaviours. \u003csup\u003e22\u003c/sup\u003e Given the fact that alkaline metals possess the largest atomic radii in the periodical table \u003csup\u003e23\u003c/sup\u003e and have the potential to form complexes with transition metal behaviors, it is possible to form alkaline-metal-doped cyclo[n]carbon complexes (\u003cem\u003en\u003c/em\u003e\u0026thinsp;\u0026ge;\u0026thinsp;14) or their boron-substituted derivatives with CN\u0026thinsp;\u0026ge;\u0026thinsp;14 if the alkaline metal center and its ligand are chosen properly to match both geometrically and electronically.\u003c/p\u003e \u003cp\u003eKeeping the inspirations in mind, using the experimentally observed perfect planar ring-like \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and theoretically predicted \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e as ligands and based on extensive global minimum searches augmented with first-principles theory calculations, we predict in this work the perfect planar alkaline-metal-centered \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) which have the record coordination numbers of CN\u0026thinsp;=\u0026thinsp;18 and 14 in planar species, respectively. Cs and Na with the atomic radii of r\u003csub\u003eCs\u003c/sub\u003e = 2.65 \u0026Aring; and r\u003csub\u003eNa\u003c/sub\u003e = 1.86 \u0026Aring; \u003csup\u003e13\u003c/sup\u003e prove to match the \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e ligands perfectly both geometrically and electronically, respectively. Effective in-plane (π-\u003cem\u003es\u003c/em\u003e)σ, (π-\u003cem\u003ep\u003c/em\u003e)σ, and (π-\u003cem\u003ed\u003c/em\u003e)σ coordination bonds are formed to dominate the attractive interactions in these novel complexes in which the alkaline-metal centers exhibit transition metal behaviors. The iso-chemical shielding surfaces and out-of-plane π and in-plane σ ring current maps of the concerned species are computationally simulated to evidence their σ\u0026thinsp;+\u0026thinsp;π dual aromaticity.\u003c/p\u003e\n\u003ch3\u003eComputational Procedures\u003c/h3\u003e\n\u003cp\u003eExtensive global-minimum (GM) searches were performed on Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003eB, Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003eB, and Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e using the TGmin2 code\u003csup\u003e24\u003c/sup\u003e at DFT level based on the basin-hopping algorithm.\u003csup\u003e25\u003c/sup\u003e Over 1000 stationary points were explored for each species at PBE/DZVP level employing the CP2K program.\u003csup\u003e26,27\u003c/sup\u003e The low-lying isomers were then fully optimized at both M06-2X and ωB97XD levels\u003csup\u003e28,29\u003c/sup\u003e with vibrational frequencies checked, with the aug-cc-pvtz basis set for C, B, Na, and K and Stuttgart relativistic small-core pseudopotentials\u003csup\u003e30,31\u003c/sup\u003e for Rb, Cs, and Fr, using the Gaussian16 program.\u003csup\u003e32\u003c/sup\u003e The fact that M06-2X produces essentially the same polyynic \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e9h\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e7h\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e structures (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) as that obtained at the more accurate QMC and CCSD levels \u003csup\u003e9,10\u003c/sup\u003e evidences the reliability of the optimized geometries depicted in Figure.1. Natural bonding orbital (NBO) analyses were implemented using NBO 7.0 program.\u003csup\u003e33\u003c/sup\u003e The energy decomposition analyses (EDA) together with the natural orbitals for chemical valence (NOCV) calculations, denoted as EDA-NOCV, \u003csup\u003e34,36\u003c/sup\u003e were carried out with the ADF program package\u003csup\u003e37\u003c/sup\u003e at M06-2X/TZ2P\u003csup\u003e38\u003c/sup\u003e level where scalar relativistic effects were considered for Cs using the zero order regular approximation (ZORA).\u003csup\u003e39\u003c/sup\u003e The frozen core approximation was not employed in EDA-NOCV computations. The overall interaction energy (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eint\u003c/sub\u003e) between two fragments is divided into three main terms: the electrostatic interaction energy (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eelstat\u003c/sub\u003e), Pauli repulsion (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003ePauli\u003c/sub\u003e), and orbital interaction energy (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e) in Eq.\u0026nbsp;(1):\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eint\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eelstat\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003ePauli\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e. (1)\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eDetailed bonding analyses on \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e), \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e), and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003eB (2) were implemented using the adaptive natural density partitioning (AdNDP 2.0) approach\u003csup\u003e40,41\u003c/sup\u003e at the M06-2X/6-31G level, with the isosurface maps of the orbitals visualized using the Visual Molecular Dynamics (VMD) software.\u003csup\u003e42\u003c/sup\u003e The iso-chemical shielding surfaces (ICSSs) \u003csup\u003e43,44\u003c/sup\u003eand isosurfaces of localized orbital locators (LOL) \u003csup\u003e45\u003c/sup\u003e were obtained with Multiwfn 3.8 code.\u003csup\u003e46\u003c/sup\u003e The anisotropy of the current-induced density (ACID) analyses were realized by the ACID code,\u003csup\u003e47\u003c/sup\u003e with the maps finally generated by POV-Ray render.\u003csup\u003e48\u003c/sup\u003e\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cb\u003eStructures and Stability\u003c/b\u003e The optimized GM structures of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003eB (\u003cb\u003e2\u003c/b\u003e), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u003cb\u003e3\u003c/b\u003e), \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003eB (\u003cb\u003e5\u003c/b\u003e), and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u003cb\u003e6\u003c/b\u003e) are collectively plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, with more alternative isomers summarized in Figures S3-S8. Figure S2 depicts the optimized GM structures of (a) the alkaline-metal-doped cyclo[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]carbon complexes M\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with M\u0026thinsp;=\u0026thinsp;Li, Na, K, Rb, Cs, and Fr and (b) alkaline-metal-doped cyclo[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]carbon derivatives M\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with M\u0026thinsp;=\u0026thinsp;Li, Na, and K at M06-2X. It is noticed that the alkaline metal atoms in the GMs are all located inside the cyclo[n]carbon rings, with the alkaline metal atoms severely off-centered in \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Li\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e K\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Li\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and slightly off-centered in \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Rb\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Fr\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e. The K atom in \u003cem\u003eC\u003c/em\u003e\u003csub\u003e7\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e K\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e lies about 1.14 \u0026Aring; above the ligand plane along the \u003cem\u003eC\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e molecular axis due to its large atomic radius (r\u003csub\u003eK\u003c/sub\u003e = 2.32 \u0026Aring;) which appears to be too big to be hosted inside the C\u003csub\u003e14\u003c/sub\u003e ring.\u003c/p\u003e \u003cp\u003eEncouragingly, Cs proves to have the right atomic radius of r\u003csub\u003eCs\u003c/sub\u003e = 2.65 \u0026Aring; to be coordinated exactly at the center of the \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e ligand in \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) to achieve the highest coordination number of CN\u0026thinsp;=\u0026thinsp;18 reported to date. As the well-defined GM of the complex (Fig. S3), Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) exhibits the alternating bond lengths of r\u003csub\u003eC\u0026ndash;C\u003c/sub\u003e = 1.343 \u0026Aring; and r\u003csub\u003eC\u0026equiv;C\u003c/sub\u003e = 1.224 \u0026Aring; at M06-2X which are well inherited from its parent ligand \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e ligand with r\u003csub\u003eC\u0026ndash;C\u003c/sub\u003e = 1.343 \u0026Aring; and r\u003csub\u003eC\u0026equiv;C\u003c/sub\u003e = 1.223 \u0026Aring; at the same theoretical level (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). The large calculated HOMO-LUMO gap of Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003egap\u003c/sub\u003e = 5.38 eV at M06-2X well supports its high chemical stability. The second isomer \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with a Cs\u003csup\u003e+\u003c/sup\u003e located outside the C\u003csub\u003e18\u003c/sub\u003e ring and the seventh isomer \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with a Cs\u003csup\u003e+\u003c/sup\u003e inserted into the C\u003csub\u003e18\u003c/sub\u003e ring appear to be 0.38 eV and 4.79 eV less stable than \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e GM at M06-2X, respectively (Fig. S3). The slightly off-centered planar \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Rb\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Fr\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e also possess the coordination numbers of CN\u0026thinsp;=\u0026thinsp;18 (Fig. S2). Both the planar neutral \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003eB (\u003cb\u003e2\u003c/b\u003e) which is isoelectronic with Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) with obviously bond-length alternations and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (3) with roughly averaged bond lengths are the well-defined GMs of the systems with CN\u0026thinsp;=\u0026thinsp;18 and 17, respectively (Fig. S4 and Fig. S5). However, the severely off-centered \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Li\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e K\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with obvious smaller alkaline metal centers Li, Na, and K appear to have much smaller coordination numbers with CN\u0026thinsp;=\u0026thinsp;4\u0026thinsp;~\u0026thinsp;6 (Fig. S2).\u003c/p\u003e \u003cp\u003eSimilarly, Na appears to have the right atomic radius (r\u003csub\u003eNa\u003c/sub\u003e = 1.86 \u0026Aring;) to be hosted exactly at the center of the \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e ligand to form the perfect planar polyynic \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) (Fig. S6) with CN\u0026thinsp;=\u0026thinsp;14. The second lowest-lying isomer \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e with Na\u003csup\u003e+\u003c/sup\u003e outside the C\u003csub\u003e14\u003c/sub\u003e ring lies only 0.23 eV higher than Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) (Fig. S6). The two close-lying lowest-lying isomers of Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e discussed above (Fig. S3 and Fig. S6) may transform between each other with low energy barriers under certain conditions. Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) as the GM of the system has the alternating bond lengths of r\u003csub\u003eC\u0026ndash;C\u003c/sub\u003e = 1.326 \u0026Aring; and r\u003csub\u003eC\u0026equiv;C\u003c/sub\u003e = 1.240 \u0026Aring; at M06-2X well comparable with the corresponding values of r\u003csub\u003eC\u0026ndash;C\u003c/sub\u003e = 1.324 \u0026Aring; and r\u003csub\u003eC\u0026equiv;C\u003c/sub\u003e = 1.237 \u0026Aring; calculated for \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e at the same theoretical level (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), while Li with the atomic radius of r\u003csub\u003eLi\u003c/sub\u003e = 1.52 \u0026Aring; proves to be too small and K with r\u003csub\u003eK\u003c/sub\u003e = 2.32 \u0026Aring; appears to be too big to be hosted at the ring center of the C\u003csub\u003e14\u003c/sub\u003e ligand, they form severely off-centered and off-planed structures, respectively (Fig. S2). With the HOMO-LUMO gap of Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003egap\u003c/sub\u003e = 5.87 eV, Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (4) is expected to have high chemical stability. The slightly off-centered planar \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003eB (\u003cb\u003e5\u003c/b\u003e) with CN\u0026thinsp;=\u0026thinsp;14 and vibrationally averaged \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (\u003cb\u003e6\u003c/b\u003e) with CN\u0026thinsp;=\u0026thinsp;13 with roughly averaged bond lengths also appear to be the well-defined GMs of the systems (Fig. S7 and Fig. S8).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs expected, the high-symmetry Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) exhibit highly characteristic calculated vibrational spectroscopic features as shown in Fig. S9, with the former possessing well characterized IR peaks at 513 and 2202 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and Raman active vibrations at 1792 and 2293 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, while the latter having two well separated IR peaks at 545 and 2160 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and one dominant Raman feature at 1252 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Such well-defined spectral features can help facilitate future experimental characterizations of these species.\u003c/p\u003e \u003cp\u003e \u003cb\u003eEDA-NOCV Bonding Scheme Analyses\u003c/b\u003e To shed insights into the bonding nature of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e), detailed EDA-NOCV analyses were carried out at M06-2X/TZ2P. The \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e3h\u003c/em\u003e\u003c/sub\u003e subgroup was applied to \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e9h\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) because the highest point group supported by ADF program is \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e8h\u003c/em\u003e\u003c/sub\u003e. It was found that Cs\u003csup\u003e+\u003c/sup\u003e and C\u003csub\u003e18\u003c/sub\u003e as the most possible reacting fragments give the most favorite interaction energy of Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eint\u003c/sub\u003e = -15.22 kcal/mol for Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) in different fragmental schemes (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). They are thus chosen as interacting species to demonstrate the bonding scheme of Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). Similarly, Na\u003csup\u003e+\u003c/sup\u003e and C\u003csub\u003e14\u003c/sub\u003e as reacting fragments with Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eint\u003c/sub\u003e = -24.44 kcal/mol are chosen for Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b).\u003c/p\u003e \u003cp\u003eThe bonding molecular orbitals (MOs) 15a\u003csub\u003e1\u003c/sub\u003e\u0026rsquo;, 19e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; and 20e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e3\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e representing covalent bonding MOs between Cs\u003csup\u003e+\u003c/sup\u003e and C\u003csub\u003e18\u003c/sub\u003e are connected with the corresponding fragmental orbitals by bold dashed lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a), with the orbital compositions tabulated in Table S2. The non-degenerate 15a\u003csub\u003e1\u003c/sub\u003e\u003csup\u003e\u0026rsquo;\u003c/sup\u003e mainly originates from the occupied 8a\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; of C\u003csub\u003e18\u003c/sub\u003e with in-plane π characteristics (abbreviated as π\u003csub\u003ein\u003c/sub\u003e) and vacant 6\u003cem\u003es\u003c/em\u003e of Cs\u003csup\u003e+\u003c/sup\u003e by (π-6\u003cem\u003es\u003c/em\u003e)σ coordination interactions, the doubly degenerate 19e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; is composed of the occupied in-plane 13e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; (π\u003csub\u003ein\u003c/sub\u003e) of C\u003csub\u003e18\u003c/sub\u003e with one nodal plane and vacant 7\u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e and 7\u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e of Cs\u003csup\u003e+\u003c/sup\u003e by (π-7\u003cem\u003ep\u003c/em\u003e)σ coordination, while the doubly degenerate 20e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; is composed of the occupied 14e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; of C\u003csub\u003e18\u003c/sub\u003e with π\u003csub\u003ein\u003c/sub\u003e characteristics with two nodal planes and vacant \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({5d}_{xy}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({5d}_{{x}^{2}-{y}^{2}}\\)\u003c/span\u003e\u003c/span\u003e of Cs\u003csup\u003e+\u003c/sup\u003e by (π-5\u003cem\u003ed\u003c/em\u003e)σ coordination. As detailed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, EDA analyses demonstrate that the overall interaction energy of Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eint\u003c/sub\u003e = -15.22 kcal/mol between the Cs\u003csup\u003e+\u003c/sup\u003e and C\u003csub\u003e18\u003c/sub\u003e in Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e consists of the Pauli repulsion Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003ePauli\u003c/sub\u003e = 1.89 kcal/mol, Coulombic attraction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eelstat\u003c/sub\u003e = -3.16 kcal/mol, and orbital interaction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e = -13.95 kcal/mol, with covalent orbital interaction making a dominating contribution of 81.5% to the overall attraction interaction (-17.11 kcal/mol), while electrostatic attraction contributing only 18.5%. The decompositions of the orbital interactions Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e into pairwise contributions between occupied and vacant MOs of the fragments provide quantitative insight into the charge flow. The strongest orbital interaction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(1)\u003c/sub\u003e (20e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo;, 27.5%) arises mainly from [C\u003csub\u003e18\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e)] \u0026rarr; [Cs\u003csup\u003e+\u003c/sup\u003e (5\u003cem\u003ed\u003c/em\u003e)] where C\u003csub\u003e18\u003c/sub\u003e serves as a π\u003csub\u003ein\u003c/sub\u003e-donor to coordinate the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({5d}_{xy}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({5d}_{{x}^{2}-{y}^{2}}\\)\u003c/span\u003e\u003c/span\u003e orbitals of the Cs\u003csup\u003e+\u003c/sup\u003e as σ-acceptors. The orbital interaction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(2)\u003c/sub\u003e (19e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo;, 16.8%) originates from [C\u003csub\u003e18\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e)] \u0026rarr; [Cs\u003csup\u003e+\u003c/sup\u003e (7\u003cem\u003ep\u003c/em\u003e)] where the 7\u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e and 7\u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e orbitals of the Cs\u003csup\u003e+\u003c/sup\u003e serve as σ-acceptors. The orbital interaction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(3)\u003c/sub\u003e (15a\u003csub\u003e1\u003c/sub\u003e\u0026rsquo;, 14.1%) originates from [C\u003csub\u003e18\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e)] \u0026rarr; [Cs\u003csup\u003e+\u003c/sup\u003e (6\u003cem\u003es\u003c/em\u003e)] where the 6\u003cem\u003es\u003c/em\u003e orbital of the Cs\u003csup\u003e+\u003c/sup\u003e is a σ-acceptor. Fig. S10 shows the corresponding deformation densities Δ\u003cem\u003eρ\u003c/em\u003e associated with the pairwise interactions Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(1)\u003c/sub\u003e, Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(2)\u003c/sub\u003e and Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(3)\u003c/sub\u003e in Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, further indicating that C\u003csub\u003e18\u003c/sub\u003e serves as a π\u003csub\u003ein\u003c/sub\u003e-donor while Cs\u003csup\u003e+\u003c/sup\u003e is a σ-acceptor in the complex.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDetailed EDA-NOCV calculations for \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) gives a similar trend as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The bonding MOs 6a\u003csub\u003e1\u003c/sub\u003e\u003csup\u003e\u0026rsquo;\u003c/sup\u003e, 6e\u003csub\u003e1\u003c/sub\u003e\u003csup\u003e\u0026rsquo;\u003c/sup\u003e and 5e\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026rsquo;\u003c/sup\u003e representing covalent bonding interactions between the Na\u003csup\u003e+\u003c/sup\u003e and C\u003csub\u003e14\u003c/sub\u003e fragmental orbitals are connected by bold dashed lines with the corresponding fragmental orbitals, with the orbital compositions listed in Table S3. The 6a\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; mainly originates from the occupied 4a\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; of C\u003csub\u003e14\u003c/sub\u003e with π\u003csub\u003ein\u003c/sub\u003e characteristics and vacant 3\u003cem\u003es\u003c/em\u003e of Na\u003csup\u003e+\u003c/sup\u003e by (π-3\u003cem\u003es\u003c/em\u003e)σ coordination interactions, the doubly degenerate 6e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; is composed of occupied 5e\u003csub\u003e1\u003c/sub\u003e\u0026rsquo; of C\u003csub\u003e14\u003c/sub\u003e with π\u003csub\u003ein\u003c/sub\u003e characteristics and vacant 3\u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003ex\u003c/em\u003e\u003c/sub\u003e and 3\u003cem\u003ep\u003c/em\u003e\u003csub\u003e\u003cem\u003ey\u003c/em\u003e\u003c/sub\u003e of Na\u003csup\u003e+\u003c/sup\u003e by (π-3p)σ coordination, while the 5e\u003csub\u003e2\u003c/sub\u003e\u0026rsquo; is composed of C\u003csub\u003e14\u003c/sub\u003e with π\u003csub\u003ein\u003c/sub\u003e characteristics and vacant \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({3d}_{xy}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({3d}_{{x}^{2}-{y}^{2}}\\)\u003c/span\u003e\u003c/span\u003e of Na\u003csup\u003e+\u003c/sup\u003e by (π-\u003cem\u003e3d\u003c/em\u003e)σ coordination. EDA analyses (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) indicate that overall attraction interaction is overwhelmingly dominated by covalent orbital contribution (94.0%), while electrostatic attraction makes only a marginal contribution (6.0%). The decompositions of Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e into pairwise contributions between occupied and vacant MOs of the fragments reveals that the strongest orbital interaction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(1)\u003c/sub\u003e (24.9%) originates mainly from [C\u003csub\u003e14\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e)] \u0026rarr; [Na\u003csup\u003e+\u003c/sup\u003e (3\u003cem\u003ep\u003c/em\u003e)], the orbital interaction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(2)\u003c/sub\u003e (19.2%) arises mainly from [C\u003csub\u003e14\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e)] \u0026rarr; [Na\u003csup\u003e+\u003c/sup\u003e (3\u003cem\u003es\u003c/em\u003e)], while the orbital interaction Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(3)\u003c/sub\u003e (18.1%) originates from [C\u003csub\u003e14\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e)] \u0026rarr; [Na\u003csup\u003e+\u003c/sup\u003e (3\u003cem\u003ed\u003c/em\u003e). The corresponding deformation densities Δ\u003cem\u003eρ\u003c/em\u003e associated with the pairwise interactions Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(1)\u003c/sub\u003e, Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(2)\u003c/sub\u003e and Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(3)\u003c/sub\u003e in Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e in Fig. S11 clearly indicate that C\u003csub\u003e14\u003c/sub\u003e serves as a π\u003csub\u003ein\u003c/sub\u003e-donor while Na\u003csup\u003e+\u003c/sup\u003e is a σ-acceptor.\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\u003eEDA-NOCV results for Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) at the M06-2X/TZ2P-ZORA level, taking C\u003csub\u003e18\u003c/sub\u003e with Cs\u003csup\u003e+\u003c/sup\u003e and C\u003csub\u003e14\u003c/sub\u003e with Na\u003csup\u003e+\u003c/sup\u003e as interacting fragments, respectively. Energy values are given in kcal/mol.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" 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=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEnergy terms\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003einteraction\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCs\u003csup\u003e+\u003c/sup\u003e + C\u003csub\u003e18\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003einteraction\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e + C\u003csub\u003e14\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eint\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-15.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-24.44\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eelstat\u003c/sub\u003e\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-3.16 (18.5%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-1.42 (6.0%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003ePauli\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3.45\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-13.95 (81.5%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-22.41 (94.0%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(1)\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC\u003csub\u003e18\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e) donation\u0026rarr;[Cs\u003csup\u003e+\u003c/sup\u003e(5\u003cem\u003ed\u003c/em\u003e)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-3.84 (27.5%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u003csub\u003e14\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e) donation\u0026rarr;[Na\u003csup\u003e+\u003c/sup\u003e(3\u003cem\u003ep\u003c/em\u003e)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-5.58 (24.9%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(2)\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC\u003csub\u003e18\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e) donation\u0026rarr;[Cs\u003csup\u003e+\u003c/sup\u003e(7\u003cem\u003ep\u003c/em\u003e)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-2.34 (16.8%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u003csub\u003e14\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e) donation\u0026rarr;[Na\u003csup\u003e+\u003c/sup\u003e(3\u003cem\u003es\u003c/em\u003e)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-4.30 (19.2%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(3)\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC\u003csub\u003e18\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e) donation\u0026rarr;[Cs\u003csup\u003e+\u003c/sup\u003e(6\u003cem\u003es\u003c/em\u003e)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-1.96 (14.1%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC\u003csub\u003e14\u003c/sub\u003e (π\u003csub\u003ein\u003c/sub\u003e) donation\u0026rarr;[Na\u003csup\u003e+\u003c/sup\u003e(3\u003cem\u003ed\u003c/em\u003e)]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-4.06 (18.1%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eΔ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb(rest)\u003c/sub\u003e\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-5.81 (41.6%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-8.47 (37.8%)\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\u003ea\u003c/sup\u003eThe value in parentheses gives the percentage contribution to the total attractive interactions (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eelstat\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e);\u003c/p\u003e \u003cp\u003e \u003csup\u003eb\u003c/sup\u003eThe value in parentheses gives the percentage contribution to the total orbital interaction (Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eorb\u003c/sub\u003e)\u003c/p\u003e \u003cp\u003eThe EDA-NOCV results detailed above quantitatively indicate that the cyclo[n]carbon ligands serve as good π\u003csub\u003ein\u003c/sub\u003e-donors to stabilize alkaline metal centers in both Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) by donating their π\u003csub\u003ein\u003c/sub\u003e valence electrons partially to the vacant \u003cem\u003es, p\u003c/em\u003e, and \u003cem\u003ed\u003c/em\u003e orbitals of Cs\u003csup\u003e+\u003c/sup\u003e and Na\u003csup\u003e+\u003c/sup\u003e through effective in-plane (π-\u003cem\u003es\u003c/em\u003e)σ, (π-\u003cem\u003ep\u003c/em\u003e)σ, and (π-\u003cem\u003ed\u003c/em\u003e)σ coordination interactions.\u003c/p\u003e \u003cp\u003eLocalized orbital locator (LOL) is an effective space function in revealing the distributions of delocalized electrons on conjugated rings in molecules. We calculated in-plane LOL-σ, in-plane LOL-π\u003csub\u003ein\u003c/sub\u003e, and out-of-plane LOL-π\u003csub\u003eout\u003c/sub\u003e separately based on the corresponding in-plane σ MOs, in-plane π MOs, and out-of-plane π MOs of the systems, respectively. To better reflect spatial distributions of LOL-σ, LOL-π\u003csub\u003ein\u003c/sub\u003e, and LOL-π\u003csub\u003eout\u003c/sub\u003e in Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e), the color-filled maps of LOL-σ on the ring plane, LOL-π\u003csub\u003ein\u003c/sub\u003e on the ring plane, and LOL-π\u003csub\u003eout\u003c/sub\u003e 1 \u0026Aring; above the ring plane are plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) comparatively. By comparing the area colors on the maps, it can be clearly seen that both LOL-π\u003csub\u003ein\u003c/sub\u003e and LOL-π\u003csub\u003eout\u003c/sub\u003e exhibit heavy density distributions over the short C\u0026thinsp;\u0026equiv;\u0026thinsp;C bonds and light density distributions over the long C-C bonds, well supporting the alternating of triple and single bonds in different bond lengths in both polyynic Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eAdNDP Bonding Pattern Analyses\u003c/b\u003e Detailed AdNDP analyses in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c) and (d) unveil both the localized and delocalized bonds in \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) more vividly. As expected, out of the 72 valence electrons in Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e), 36 electrons form 18 equivalent 2c-2e C-C peripheral in-plane σ bonds with the occupation numbers of ON\u0026thinsp;=\u0026thinsp;2.00 |e|. The remaining 36 valence electrons are distributed in two types of chemical bonds, including 9 equivalent in-plane 3c-2e σ bonds on nine CsC\u003csub\u003e2\u003c/sub\u003e triangles with ON\u0026thinsp;=\u0026thinsp;1.83 |e| and 9 equivalent out-of-plane 2c-2e C-C π bonds with ON\u0026thinsp;=\u0026thinsp;1.83 |e|, respectively. Such a bonding pattern follows the 4\u003cem\u003eN\u003c/em\u003e\u0026thinsp;+\u0026thinsp;2 aromatic rule for σ aromaticity with \u003cem\u003eN\u003c/em\u003e\u003csub\u003eσ\u003c/sub\u003e = 4 and π aromaticity with \u003cem\u003eN\u003c/em\u003e\u003csub\u003eπ\u003c/sub\u003e = 4, respectively, making the planar complex σ\u0026thinsp;+\u0026thinsp;π dually aromatic in nature and rendering extra stability to the system, similar to the situation in the previously reported \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e.\u003csup\u003e11\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eSimilarly, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d), \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) possesses 7 equivalent 2c-2e C-C periphery in-plane σ bonds, 7 equivalent in-plane 3c-2e σ bonds on seven NaC\u003csub\u003e2\u003c/sub\u003e triangles, and 7 equivalent out-of-plane 2c-2e C-C π bonds, again following the 4\u003cem\u003eN\u003c/em\u003e\u0026thinsp;+\u0026thinsp;2 aromatic rule with \u003cem\u003eN\u003c/em\u003e\u003csub\u003eσ\u003c/sub\u003e = \u003cem\u003eN\u003c/em\u003e\u003csub\u003eπ\u003c/sub\u003e = 3 for σ\u0026thinsp;+\u0026thinsp;π dual aromaticity. Similar bonding patterns exist in \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003eB (\u003cb\u003e2\u003c/b\u003e) (Fig. S12). The dual aromaticities of both Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) are also well supported by their delocalized in-plane σ MOs and delocalized out-of-plane π MOs shown in Fig. S13.\u003c/p\u003e \u003cp\u003eThe simulated ICSS isosurfaces of \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) based on the ZZ components of the calculated nuclear-independent chemical shifts (NICS-ZZ) are presented as Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), in comparison with that of the previously reported σ\u0026thinsp;+\u0026thinsp;π dually aromatic \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e. It can be clearly seen that, similar to \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e, both \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e) are aromatic in nature, with the spaces inside the cyclo[n]carbon rings and within ~\u0026thinsp;1.0 \u0026Aring; above the ring planes belonging to chemical shielding areas with negative NICS-ZZ values (highlighted in yellow) and the blet-like regions around the cyclo[n]carbon rings in horizontal direction belonging to chemical deshielding areas with positive NICS-ZZ values (highlighted in green).\u003c/p\u003e \u003cp\u003eThe widely used ACID method can be employed to display graphically the ring currents induced by an external magnetic field in vertical directions perpendicular to the cyclo[n]carbon ring. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) presents the calculated out-of-plane π and in-plane σ ring currents maps for both \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e), in comparison with the corresponding ring currents obtained for \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e at the same theoretical level, respectively. As clearly indicated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), these alkaline-metal-centered polyynic complex monocations do possess intrinsic σ aromaticity and π aromaticity simultaneously, similar to their neutral parent ligands \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e in ring current distributions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, based on extensive first-principles theory calculations, we have predicted in this work a series of alkaline-metal-doped complexes Cs\u0026copy;C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e1\u003c/b\u003e), Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003eB (\u003cb\u003e2\u003c/b\u003e), Cs\u0026copy;C\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u003cb\u003e3\u003c/b\u003e), Na\u0026copy;C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cb\u003e4\u003c/b\u003e), Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003eB (\u003cb\u003e5\u003c/b\u003e), and Na\u0026copy;C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u003cb\u003e6\u003c/b\u003e) which turn out to be GMs of the systems with the record coordination numbers of CN\u0026thinsp;=\u0026thinsp;18\u0026thinsp;~\u0026thinsp;13 in planar species. These σ\u0026thinsp;+\u0026thinsp;π dually aromatic complexes possess effective in-plane (π-\u003cem\u003es\u003c/em\u003e)σ, (π-\u003cem\u003ep\u003c/em\u003e)σ, and (π-\u003cem\u003ed\u003c/em\u003e)σ coordination interactions which dominate the attractive interaction between the alkaline metal center as σ-acceptor and its cyclo[n]carbon ligand as in-plane π-donor, evidencing the transition metal behaviors of the alkaline metal centers in them. Similar to the situation in the recently observed alkaline-earth metal carbonyl species,\u003csup\u003e21\u003c/sup\u003e the perfect planar alkaline metal-centered polyynic cyclo[n]carbon complexes proposed in this work (n\u0026thinsp;=\u0026thinsp;18, 14) with relatively low coordination energies may be produced in gas phases by laser ablation of alkaline-metal-carbon mixed binary targets and characterized by spectroscopic measurements at low temperatures to further push the boundary of coordination chemistry.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere are no conflicts to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe work was supported by the National Natural Science Foundation of China (21720102006, 21973057 and 22003034) and Natural Science Foundation of Shanxi Province of China (20210302124002).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eS.D.Li, Z.H Wei, and Q, Chen conceived the project and finalized the manuscript. M.Z and R.N.Y did the calculations and prepared the first draft. Y.B.Wu helped analyze the data. All authors approved the final version.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKroto, H. W., Heath, J. R., O\u0026rsquo;Brien, S. C., Curl, R. F. \u0026amp; Smalley, R. E. C\u003csub\u003e60\u003c/sub\u003e: Buckminsterfullerene. Nature 318, 162\u0026ndash;163 (1985).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eIijima S. Helical Microtubules of Graphitic Carbon. Nature 354, 56\u0026ndash;68 (1991).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNovoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., Grigorieva, I. V. \u0026amp; Firsov, A. A. Electric field effect in atomically thin carbon films. Science 306, 666\u0026ndash;669 (2004).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKaiser, K., Scriven, L. 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Rev. 105, 3758\u0026ndash;3772 (2005).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePovray, Persistence of vision raytracer, POV-Ray 3.7, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.povray.org/\u003c/span\u003e\u003cspan address=\"http://www.povray.org/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-2614379/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2614379/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSearching for the maximum coordination number (CN) in planar species with novel bonding patterns has fascinated chemists for many years. Using the experimentally observed cyclo[18]carbon \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e18\u003c/sub\u003e and theoretically predicted cyclo[14]carbon \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e C\u003csub\u003e14\u003c/sub\u003e as effective ligands and based on extensive first-principles theory calculations, we predict herein their perfect planar alkaline-metal-doped complexes \u003cem\u003eD\u003c/em\u003e\u003csub\u003e9\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e1\u003c/strong\u003e) and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e7\u003c/sub\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e4\u003c/strong\u003e) which, as the global minima of the systems with an alkaline metal atom located exactly at the center, possess the record coordination numbers of CN = 18 and 14 in planar species, respectively. More interestingly, detailed energy decomposition and adaptive natural density partitioning bonding analyses indicate that the hypercoordinate alkaline-metal centers in these σ + π dually aromatic complexes exhibit obvious transition metal behaviors, with effective in-plane (π-6\u003cem\u003es\u003c/em\u003e)σ, (π-7\u003cem\u003ep\u003c/em\u003e)σ, and (π-5\u003cem\u003ed\u003c/em\u003e)σ coordination bonds formed in Cs©C\u003csub\u003e18\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e1\u003c/strong\u003e) and (π-3\u003cem\u003es\u003c/em\u003e)σ, (π-3\u003cem\u003ep\u003c/em\u003e)σ, and (π-3\u003cem\u003ed\u003c/em\u003e)σ coordination interactions fabricated in Na©C\u003csub\u003e14\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003e4\u003c/strong\u003e) to dominate the overall attractive interactions between the metal center and its cyclo[n]carbon ligand. Similar dually aromatic alkaline-metal-centered planar \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e17\u003c/sub\u003eB (\u003cstrong\u003e2\u003c/strong\u003e), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Cs©C\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (\u003cstrong\u003e3\u003c/strong\u003e), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e13\u003c/sub\u003eB (\u003cstrong\u003e5\u003c/strong\u003e), and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e Na©C\u003csub\u003e13\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (\u003cstrong\u003e6\u003c/strong\u003e) have also been obtained with CN = 18, 17, 14, and 13, respectively.\u003c/p\u003e","manuscriptTitle":"Perfect planar polyynic cyclo[n]carbon complexes [Cs©C18]+ and [Na©C14]+ with alkaline-metal centers exhibiting record coordination numbers and transition metal behaviors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-03-01 16:03:41","doi":"10.21203/rs.3.rs-2614379/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"36c72da1-1475-44fb-8ecb-06297727e7a8","owner":[],"postedDate":"March 1st, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":19585564,"name":"Physical sciences/Chemistry/Coordination chemistry"},{"id":19585565,"name":"Physical sciences/Chemistry/Inorganic chemistry"},{"id":19585566,"name":"Physical sciences/Chemistry/Materials chemistry"},{"id":19585567,"name":"Physical sciences/Chemistry/Theoretical chemistry"}],"tags":[],"updatedAt":"2023-06-10T08:59:35+00:00","versionOfRecord":[],"versionCreatedAt":"2023-03-01 16:03:41","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2614379","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2614379","identity":"rs-2614379","version":["v1"]},"buildId":"_2-kVJe1T_tPrBINL-cwx","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}