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

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.