Electronic Structure of Ni₂E₂ Complexes (E = S, Se, Te) and a Global Analysis of M₂E₂ Compounds: A Case for Quantized E₂^n– Oxidation Levels with n = 2, 3, or 4

The diamagnetic compounds Cp′₂Ni₂E₂ (1: E = S, 2: E = Se, 3: E = Te; Cp′ = 1,2,3,4,-tetraisopropylcyclopentadienyl), first reported by Sitzmann and co-workers in 2001 [Sitzmann, H.; Saurenz, D.; Wolmershauser, G.; Klein, A.; Boese, R. Organometallics 2001, 20, 700], have unusual E···E distances, leading to ambiguities in how to best describe their electronic structure. Three limiting possibilities are considered: case A, in which the compounds contain singly bonded E₂^2– units; case B, in which a three-electron E∴E half-bond exists in a formal E₂^3– unit; case C, in which two E^2– ions exist with no formal E–E bond. One-electron reduction of 1 and 2 yields the new compounds [Cp*₂Co]­[Cp′₂Ni₂E₂] (1red: E = S, 2red: E = Se; Cp* = 1,2,3,4,5-pentamethylcyclopentadieyl). Evidence from X-ray crystallography, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy suggest that reduction of 1 and 2 is Ni-centered. Density functional theory (DFT) and ab initio multireference methods (CASSCF) have been used to investigate the electronic structures of 1–3 and indicate covalent bonding of an E₂^3– ligand with a mixed-valent Ni₂(II,III) species. Thus, reduction of 1 and 2 yields Ni₂(II,II) species 1red and 2red that bear unchanged E₂^3– ligands. We provide strong computational and experimental evidence, including results from a large survey of data from the Cambridge Structural Database, indicating that M₂E₂ compounds occur in quantized E₂ oxidation states of (2 × E^2–), E₂^3–, and E₂^2–, rather than displaying a continuum of variable E–E bonding interactions.