Electronic Structure of Ni_2E_2 Complexes (E = S, Se, Te) and a Global Analysis of M_2E_2 Compounds: A Case for Quantized E_2~(n-) Oxidation Levels with n = 2, 3, or 4

摘要

The diamagnetic compounds Cp'_2Ni_2E_2 (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.; Wolmer-shauser, 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 E_2~(2-) units; case B, in which a three-electron E∴E half-bond exists in a formal E_2~(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'_2Ni_2E_2] (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 E_2~(3-) ligand with a mixed-valent Ni_2(Ⅱ,Ⅲ) species. Thus, reduction of 1 and 2 yields Ni_2(Ⅱ,Ⅱ) species 1red and 2red that bear unchanged E_2~(3-) ligands. We provide strong computational and experimental evidence, including results from a large survey of data from the Cambridge Structural Database, indicating that M_2E_2 compounds occur in quantized E_2 oxidation states of (2 × E~(2-)), E_2~(3-), and E_2~(2-), rather than displaying a continuum of variable E-E bonding interactions.