Unusual platinum(III) valence state stabilized through the iodide-bridged three-center four-electron bonding

Abstract One of the goals in synthetic chemistry is to obtain compounds featuring unusual valence states that are stable under ambient conditions. At present, stabilizing unusual Pt(III) states is considered difficult, except through direct Pt–Pt bonding such as that in platinum-blues or organometallization using bulky ligands. Pt(III) stabilization is also very difficult in halogen-bridged metal complex chains (MX-Chains). Herein, the iodide-bridged Pt(III) dimer compound [Pt2(en)4I3]I3 (3) (en = ethylenediamine), which is prepared by iodine oxidation of [PtII(en)2] (1), has been successfully synthesized and characterized. This compound is stable and is obtained as diamond-shaped single crystals with a lustrous emerald-green color under reflected light and a red color under transmitted light. The two Pt centers are crystallographically equivalent and can be assigned as Pt(III). Density functional theory calculations reveal that the Pt(III) state is stabilized by the three-center four-electron (3c-4e) bonding in the Pt–I–Pt core (dz2–pz–dz2), in addition to the very strong antiferromagnetic state. This type of bonding stabilization does not appear in MX-Chains. The experimental polarized reflection spectra show the Pt(III) state character and are in agreement with the simulated spectra. The stabilization of Pt(III) through a 3c-4e bonding mechanism is novel; thus, this work provides new insight for stabilizing the unusual Pt(III) state.