Electronic structure of metal clusters. 1. Photoelectron spectra and molecular orbital calculations on alkylidynetricobalt nonacarbonyl clusters
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Ultraviolet photoelectron (PE) spectra and molecular orbital (MO) calculations are reported for several (alkylidyne)tricobalt nonacarbonyl compounds, RCCo3(CO)9, where R = H, CH3, OCH3, Cl, Br, and I. The molecular ionization energies from the PE spectra are closely related to the valence-orbital electronic structure. The MO calculations are helpful in assigning the spectra, in confirming the interpretation of trends, and in providing a consistent description of the electronic structure. The lowest energy ionization corresponds to a delocalized orbital which is both Co-C and Co-Co bonding. This ionization is followed closely by bands due to the Co-Co bonds and Co-CO π bonds. Well separated from these ionizations are those which correspond to Co-C bonding orbitals. The origin of this molecular orbital pattern is described both as the joining of three Co(CO)3 fragments with a RC fragment and as the perturbation of a CCo3 cluster by nine CO ligands and one R ligand. Although both descriptions are equally valid, the former provides a simpler interpretation of the MO results and PE spectra. Comparison of the PE spectra of H3CX, HC=CX, C6H5X, and (CO)9Co3CX for the halogens suggests that the bonding of the cobalt cluster to the apical C is much closer to HC=CX or C6H5X than it is to H3CX. Both the PE spectra and MO calculations suggest that the apical carbon is electron rich but that the π system is sufficiently flexible to act as either a donor or acceptor. A localized MO description with an sp-hybridized C, in which a lone pair forms a dative bond to the metal triangle and the remaining p orbitals form multicentered π bonds to the Co3 system, is consistent with all experimental evidence. If these ideas are extended to other systems, the hybridization at any carbon atom is determined primarily by the geometry of its non-transition-metal substituents. Thus, the CR group is best described as sp hybridized regardless of whether it is bound to a single metal or bridging three metals. © 1981, American Chemical Society. All rights reserved.
author list (cited authors)
Chesky, P. T., & Hall, M. B.