The binuclear cyclopentadienylvanadium carbonyls (η⁵-C ₅H₅)₂V₂(CO)₇ and (η⁵-C₅H₅)₂V₂(CO) ₆: Comparison with homoleptic chromium carbonyls

The cyclopentadienylvanadium carbonyls CpV(CO)n (n = 4, 3, 2, 1), Cp ₂V₂(CO)₇, and Cp₂V ₂(CO)₆ have been studied by density functional theory using the B3LYP and BP86 functionals. The optimized structure of CpV(CO) ₄ is close to that found experimentally by X-ray and electron diffraction methods. For the coordinately unsaturated CpV(CO)_n (n = 3, 2, 1) the computed v(CO) frequencies by BP86 are related to those found in low-temperature matrix isolation experiments but show some significant differences for n = 3 and 2 in band spacings suggesting some matrix effects on the geometries. An optimized structure for Cp₂V₂(CO) ₇ is found with one symmetrical bridging carbonyl group and a V-V distance of 3.306 Å (BP86) suggesting a single bond. Energetically competitive structures for Cp₂V₂(CO)₆ include a doubly symmetrically bridged singlet structure with only two-electron donor carbonyl groups, a triplet doubly semibridged structure, and a singlet structure with one four-electron donor bridging carbonyl group. Thermodynamic considerations rationalize why neither Cp₂V₂(CO) ₇ nor Cp₂V₂(CO)₆ have yet been synthesized. Thus Cp₂V₂(CO)₇ is predicted to be unstable with respect to dissociation into CpV(CO)₄ + CpV(CO) ₃. Furthermore, the dissociation energy of the lowest lying singlet isomer from Cp₂V₂(CO)₆ to Cp₂V ₂(CO)₅ is only 10.8 kcal/mol (BP86) consistent with the formation of Cp₂V₂(CO)₅ rather than Cp ₂V₂(CO)₆ upon decomposition of the presumed CpV(CO)₃H₂ dihydride generated by acidification of the dianion CpV(CO)₃^2-.