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

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[dbnd]V distance in this structure of 2.696 Å is close to the experimental distance of 2.733 Å as determined by X-ray crystallography. The Wiberg bond index (WBI) of 0.94 supports the formulation of this V[dbnd]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[tbnd]V distance of 2.374 Å 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 Å 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.