Binuclear vanadium dimethylphosphino carbonyls: vanadium-vanadium multiple bonds and four-electron donor carbonyl groups as structural features in unsaturated systems

Abstract

Abstract The structures and energetics of the binuclear dimethylphosphido vanadium carbonyls (Me2P)2V2(CO)n (n = 10, 9, 8, 7, 6) have been examined using density functional theory. The experimentally known (Me2P)2V2(CO)8 with two bridging Me2P groups and all terminal CO groups is found to be the lowest energy structure by a substantial margin. The predicted V V distance in this structure of 2.696 A is close to the experimental distance of 2.733 A as determined by X-ray crystallography. The Wiberg bond index (WBI) of 0.94 supports the formulation of this V V interaction as a formal double bond, consistent with the 18-electron rule. Decarbonylation of (Me2P)2V2(CO)8 is predicted to give a singly CO-bridged (Me2P)2V2(CO)6(µ-CO) structure with a shorter V V distance of 2.374 A corresponding to a WBI of 1.30 and suggesting a formal triple bond. Further decarbonylation to (Me2P)2V2(CO)6 is predicted to lead to structures with four-electron donor bridging η2-µ-CO groups rather than structures with formal V V quadruple bonds. For the carbonyl-richer system (Me2P)2V2(CO)9 the lowest energy structure retains two bridging Me2P ligands and all terminal CO groups. Its V V distance of 3.138 A coupled with a WBI value of 0.23 suggests the formal single bond required by the 18-electron rule. The lowest energy (Me2P)2V2(CO)10 structure has a similar V V single bond. However, only one of its Me2P ligands bridges the central V V bond; the other Me2P group is a terminal ligand bonded to a single vanadium atom. Structures with intact Me2P-PMe2 ligands having direct P P bonds are very high energy structures relative to their isomers with two separate Me2P units lacking a direct P P bond.