The nature of the transition-metal carbonyl bond and molecular orbital calculations and ultraviolet photoelectron spectroscopy of hydrido transition-metal carbonyl clusters

Abstract

The interaction coordinates of chromium hexacarbonyl were quantitatively predicted by the overlap populations from Fenske-Hall molecular orbital (FHMO) calculations. The trans effect, in the interaction coordinates, was shown to be consistent with strong chromium-carbon (pi) bonding. An approximate dissociation curve for the dissociation of a single carbonyl ligand (CO) from chromium hexacarbonyl was calculated by the Hartree-Fock-Roothaan method. The effect of changes in chromium-carbon bond distance on the total density, on the atomic deformation density, on the fragment deformation density, on the Mulliken populations, and on the force between the carbon and oxygen, all indicated that at extremely long chromium-carbon bond distances, CO will act as a "(sigma) only" donor which will result in a C-O bond stronger than that in free CO. This result is consistent with a few carbonyl complexes which have either a CO bond distance shorter than that of free CO or a CO stretching frequency higher than that of free CO. The chromium-carbonyl bond was found to consist of a rehybridization of carbon "lone pair" density into the chromiun-carbon interbond region, as opposed to the classical description of carbonyl to chromium (sigma) donation, and of a chromium to carbonyl (pi) back-donation which does involve direct charge transfer and direct Cr-C bonding. Approximately 24% of the dissociation energy of a chromium-carbon bond is from (pi) orbital contributions. A limited configuration interaction calculation demonstrated that the near-degenerate correlation energy is (pi)-dominated. Fenske-Hall MO calculations and ultraviolet photoelectron (PE) spectroscopy were performed on hydro-, chloro- and bromo-nonacarbonyl-tri-(mu)-hydrido-(mu)(,3)-methylidyne-triangulo-triruthenium clusters and decacarbonyl-di-(mu)-hydrido-triangulo-triosmium. The ruthenium clusters, which have single hydrogen bridges, were shown to have no net ruthenium-ruthenium bonds and a methylidyne carbon atom with three C-Ru (sigma) bonds. The osmium cluster, which has a double hydrogen bridge, was shown to have retained a partial osmium-osmium bond, derived mainly from tricarbonyl-osmium fragment pseudo "lone pair" orbitals.