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