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 C_2h 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 C_2h 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 D_4d 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 C_s 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.

Keywords:

• Octacarbonyldivanadium
• Unsaturated
• Binuclear
• Metal carbonyl

Acknowledgements

We are grateful to the National Science Foundation of China and to US National Science Foundation (grant CHE-0451445) for support of this work. One of the authors (RBK) would also like to acknowledge helpful discussions with Prof. E. Jemmis of the University of Hyderabad (India).