The Binuclear Cyclopentadienylvanadium Carbonyls (η 5 ‐C 5 H 5 ) 2 V 2 (CO) 7 and (η 5 ‐C 5 H 5 ) 2 V 2 (CO) 6 : Comparison with Homoleptic Chromium Carbonyls

The cyclopentadienylvanadium carbonyls CpV(CO) n ( n = 4, 3, 2, 1), Cp 2 V 2 (CO) 7 , and Cp 2 V 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 ν(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 Cp 2 V 2 (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 Cp 2 V 2 (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 Cp 2 V 2 (CO) 7 nor Cp 2 V 2 (CO) 6 have yet been synthesized. Thus Cp 2 V 2 (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 Cp 2 V 2 (CO) 6 to Cp 2 V 2 (CO) 5 is only 10.8 kcal/mol (BP86) consistent with the formation of Cp 2 V 2 (CO) 5 rather than Cp 2 V 2 (CO) 6 upon decomposition of the presumed CpV(CO) 3 H 2 dihydride generated by acidification of the dianion CpV(CO) 3 2– .(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)