1,3-Diphosphacyclobutadiene sandwich compounds as bidentate ligands in metal carbonyl chemistry: Binuclear chromium derivatives

Abstract

The structures and thermochemistry of the binuclear 1,3-diphosphacyclobutadiene chromium carbonyl complexes (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) have been examined by density functional theory. The only low-energy structure for the hexacarbonyl (Me2C2P2)2Cr2(CO)6 consists of a bidentate chelating (η4-Me2C2P2)2Cr(CO)2 sandwich diphosphine ligand bonded to a Cr(CO)4 unit with a long ∼4.0 Å Cr⋯Cr distance indicating lack of a direct metal-metal bond. The lowest energy structure for the pentacarbonyl (Me2C2P2)2Cr2(CO)5 is a triplet structure derived from this singlet hexacarbonyl structure by loss of a CO group from the sandwiched chromium atom. The bridging η1,4-Me2C2P2 ligands found in these two structures, using the four π-electrons of the P2C2 ring to bond to one chromium atom and a phosphorus lone pair to bond to the other chromium atom but not sandwiching a chromium atom, are features of the next lowest energy triplet and singlet (Me2C2P2)2Cr2(CO)5 structures. In contrast to the hexa- and pentacarbonyls, the lowest energy structures of the more highly unsaturated tetra- and tricarbonyls (Me2C2P2)2Cr2(CO)n (n = 4, 3) have terminal tetrahapto η4-Me2C2P2 rings and chromium-chromium multiple bonds.