Interplay between two-electron and four-electron donor carbonyl groups in oxophilic metal systems: highly unsaturated...

The divanadocene carbonyls Cp2V2(CO)n (n = 5, 4, 3, 2, 1; Cp = eta5-C5H5) have been studied by density functional theory using the B3LYP and BP86 functionals. The global minimum for Cp2V2(CO)5 with a V[triple bond]V distance of 2.452 A (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[triple bond]V distance of 2.444 A (BP86). However, slightly higher energy singlet Cp2V2(CO)4 structures are found either with a V[triple bond]V distance of 2.547 A (BP86) and one four-electron donor bridging CO group or with a V[quadruple bond]V distance of 2.313 A (BP86) and all two-electron donor bridging CO groups. Comparison is made between Cp2V2(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 Cp2V2(eta2-mu-CO)2 isomers as well as triplet, quintet, and septet structures of Cp2V2(CO) with extremely low nu(CO) frequencies around 1400 cm(-1). Our most remarkable structural finding is the extremely short vanadium-vanadium distance (1.80 A, BP86) predicted for the singlet structure of Cp2V2(CO).