Octacarbonyldivanadium: a highly unsaturated binuclear metal carbonyl

Abstract

The first structural characterization of the highly unsaturated octacarbonyldivanadium, V2(CO)8, is reported using density functional theory (DFT) with the B3LYP and BP86 functionals. Various combinations of vanadium–vanadium multiple bonding, four-electron bridging CO groups, and metal electron configurations less than the 18-electron rare gas configuration are found to accommodate the high unsaturation implied by the V2(CO)8 stoichiometry. Thus, the global minimum for V2(CO)8 is a C2h structure (1) with two four-electron bridging CO groups and a vanadium–vanadium distance of 2.896 (B3LYP) or 2.858 Å (BP86), suggestive of a V–V single bond and thus 16-electron configurations for the vanadium atoms. The next lowest-lying structure (2) for V2(CO)8 at 10.6 (B3LYP) or 12.5 kcal mol−1 (BP86) above the global minimum (1) also has C2h symmetry with two bridging CO groups, but a much shorter vanadium–vanadium distance of 2.440 (B3LYP) or 2.460 Å (BP86), suggestive of a V≡V triple bond. Of similar energy to 2 is a D4d unbridged structure (3) for V2(CO)8 with an extremely short vanadium–vanadium distance of 1.941 (B3LYP) or 1.967 Å (BP86), consistent with, but not proving, the presence of the quintuple bond required to give each vanadium atom the favored 18-electron rare gas electronic configuration. The one other optimized structure for V2(CO)8 within 27 kcal mol−1 of the global minimum and without any imaginary vibrational frequencies is a highly unsymmetrical Cs structure (6) with three very unsymmetrical bridging CO groups and a vanadium–vanadium distance of 2.615 (B3LYP) or 2.509 Å (BP86), suggestive of a V=V double bond.