Interplay between two-electron and four-electron donor carbonyl groups in oxophilic metal systems: Highly unsaturated divanadocene carbonyls

The divanadocene carbonyls Cp₂V₂(CO)_n (n = 5, 4, 3, 2, 1; Cp = η⁵-C₅H₅) have been studied by density functional theory using the B3LYP and BP86 functionals. The global minimum for Cp₂V₂(CO)₅ with a V≡V distance of 2.452 Å (BP86) is essentially the same as the structure of the known Cp₂V₂(CO)₅ determined by X-ray diffraction. The global minimum of Cp₂V₂(CO)₄ is a triplet electronic state with a V≡V distance of 2.444 Å (BP86). However, slightly higher energy singlet Cp₂V₂(CO) ₄ structures are found either with a V≡V distance of 2.547 Å (BP86) and one four-electron donor bridging CO group or with a V- ⁴V distance of 2.313 Å (BP86) and all two-electron donor bridging CO groups. Comparison is made between Cp₂V ₂(CO)₃ and the recently synthesized quintuply bonded RCrCrR (R = bulky aryl group) complexes of Power and co-workers. Four-electron donor bridging carbonyl groups become more prevalent upon further decarbonylation, leading ultimately to three singlet Cp₂V ₂(η²-μ-CO)₂ isomers as well as triplet, quintet, and septet structures of Cp₂V₂(CO) with extremely low ν(CO) frequencies around 1400 cm^-1. Our most remarkable structural finding is the extremely short vanadium-vanadium distance (1.80 Å, BP86) predicted for the singlet structure of Cp₂V ₂(CO).