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

The diamagnetic compounds Cp'2Ni2E2 (1: E = S, 2: E = Se, 3: E = Te; Cp'\n= 1,2,3,4,-tetraisopropylcyclopentadienyl), first reported by Sitzmann\nand co-workers in 2001 {[}Sitzmann, H.; Saurenz, D.; Wolmershauser, G.;\nKlein, A.; Boese, R. Organometallics 2001, 20, 700], have unusual E\ncenter dot center dot center dot E distances, leading to ambiguities in\nhow to best describe their electronic structure. Three limiting\npossibilities are considered: case A, in which the compounds contain\nsingly bonded E-2(2)- units; case B, in which a three-electron E-E\nhalf-bond exists in a formal E-2(3-) unit; case C, in which two E-2(-)\nions exist with no formal E-E bond. One-electron reduction of 1 and 2\nyields the new compounds {[}Cp{*}Co-2]{[}Cp'2Ni2E2] (1red: E = S, 2red:\nE = Se; Cp{*} = 1,2,3,4,5-pentamethylcyclopentadieyl). Evidence from\nX-ray crystallography, X-ray absorption spectroscopy, and X-ray\nphotoelectron spectroscopy suggest that reduction of 1 and 2 is\nNi-centered. Density functional theory (DFT) and ab initio\nmultireference methods (CASSCF) have been used to investigate the\nelectronic structures of 1-3 and indicate covalent bonding of an E23-\nligand with a mixed-valent Ni-2(II,III) species. Thus, reduction of 1\nand 2 yields Ni2(II,II) species 1red and 2red that bear unchanged\nE-2(3-) ligands. We provide strong computational and experimental\nevidence, including results from a large survey of data from the\nCambridge Structural Database, indicating that M2E2 compounds occur in\nquantized E-2 oxidation states of (2 x E2-), E-2(3-), and E-2(2-),\nrather than displaying a continuum of variable E-E bonding interactions.