Bonding of Seven Carbonyl Groups to a Single Metal Atom: Theoretical Study of M(CO)n (M = Ti, Zr, Hf; n = 7, 6, 5, 4).

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic metal-carbonyls of the group 4 elements, M(CO)n (M = Ti, Zr, Hf; n = 7, 6, 5, 4) were predicted using density functional theory. Analogous M(CO)n structures were found for all three metals. The global minima for the 1 8-electron M(CO)7 molecules are all singlet C3v capped octahedra. The global minima for the 16-electron M(CO)6 species are triplet M(CO)6 structures distorted from Oh symmetry to D3d symmetry. However, the corresponding singlet M(CO)6 structures lie within 5 kcal/mol of the triplet global minima. The global minima for M(CO)n (n = 5, 4) are triplet structures derived from the D3d distorted octahedral structures of M(CO)6 by removal of one or two CO groups, respectively. Quintet D3h trigonal bipyramidal structures for M(CO)5 and singlet Td tetrahedral structures for M(CO)4 are also found, as well as higher energy structures for M(CO)6 and M(CO)7 containing a unique CO group bonded to the metal atom through both M-C and M-O bonds. The dissociation energies M(CO)7 → M(CO)6 + CO are substantial, indicating no fundamental problem in bonding seven CO groups to a single metal atom. [ABSTRACT FROM AUTHOR]