Binuclear 1,3-Diphosphacyclobutadiene Vanadium Carbonyls: Bending of the P2C2 Ring in an Unsaturated System with a Vanadium-Vanadium Multiple Bond

Abstract The structures and energetics of the binuclear 1,3-diphosphacyclobutadiene vanadium carbonyls of the type (Me2C2P2)2V2(CO)n have been investigated by density functional theory. The lowest energy (Me2C2P2)2V2(CO)n (n = 8, 7, 6, 5) structures are singlet spin state structures with various combinations of single and double vanadium-vanadium bonds and six-electron donor bridging η1,η4-Me2C2P2 ligands to give each vanadium atom the favored 18-electron configuration. Higher energy singlet (Me2C2P2)2V2(CO)6 structures have terminal η4-Me2C2P2 ligands and formal V≡V triple bonds similar to that found in the experimentally known binuclear cyclopentadienylvanadium carbonyl derivative (η5-C5H5)2V2(CO)5. The lowest energy structure of the highly unsaturated (Me2C2P2)2V2(CO)4 by a significant margin has an unprecedented type of eight-electron donor bent tetrahedrane-like Me2C2P2 ligand not found in the low energy structures of the later first row transition metals. The lowest energy structure of the even more highly unsaturated (Me2C2P2)2V2(CO)3 has a vanadium (η4-Me2C2P2)2V sandwich functioning as a bidentate chelating ligand to a V(CO)3 unit supplemented by a V≡V triple bond.