Carbon-hydrogen bond activation in bridging cyclobutadiene ligands in unsaturated binuclear vanadium carbonyl derivatives.

The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory (DFT). The lowest energy (C4H4)2V2(CO)8 structure consists of two C4H4V(CO)4 units linked by a V-V single bond of length 3.4 Å. The two lowest energy (C4H4)2V2(CO)7 structures also have formal V-V single bonds. The "extra" two electrons to give each vanadium atom in these heptacarbonyls the favored 18-electron configuration can come from either an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings or from a four-electron donor bridging η2-µ-CO group with a short V-O distance. The lowest energy (C4H4)2V2(CO)6 structure has a formal V≡V triple bond of length 2.52 Å similar to the V≡V triple bond of length 2.46 Å found in the experimentally known cyclopentadienyl derivative (η5-C5H5)2V2(CO)5. The lowest energy structures for the more highly unsaturated (C4H4)2V2(CO)n (n = 5, 4, 3, 2) have at least two four-electron donor bridging η2-µ-CO groups and a vanadium-vanadium bond order sufficient to give each vanadium atom at least a 16-electron configuration. The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory. The two lowest energy (C4H4)2V2(CO)7 structures include one with an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings and another with a four-electron donor bridging η2-µ-CO group with a short V-O bonding distance.