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Issue 12, 1991
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When is a vertex not a vertex? An analysis of the structures of [MB10H12] metallaboranes


There are two extreme descriptions of the bonding between metal fragments and the {B10H12} ligand. In the first the metal is regarded as a full cluster vertex in an 11 vertex nido metallaborane; the B10 residue is formally arachno-{B10H12}4–. In the second the metal is a poor cluster vertex and does not significantly perturb the borane fragment architecture, formally nido-{B10H12}2–. nido-{B10H12}2– and arachno-{B10H12}4– have exactly the same pattern of connectivities, but their structures may be distinguished by root mean square (r.m.s.) misfit calculations. Applications of these calculations to [MB10H12] metallaboranes reveals clear examples of both extreme formalisms, and in [(C6H11)3PAuB10H12] and [(OC)3CoB10H12] the formal metal oxidation states (Au+, Co3+) that follow directly from these descriptions of the {B10H12} ligand agree well with independent measurement. In addition, however, several metallaboranes are found to have structures in which the B10 residue lies between that of {B10H12}2– and {B10H12}4–. The verticity of a metal fragment is introduced as a convenient way of describing its relative degree of incorporation into the metallaborane as a true cluster vertex. By analysis of the results of extended-Hückel molecular orbital–fragment molecular orbital (EHMO–FMO) calculations verticity is found, to a first approximation, to be directly related to the number of available valence orbitals the metal fragment possesses. Metal fragments that are one-orbital sources are poor vertices, whilst those that are three-orbital sources are good vertices, but the boundary between good- and poor-metal vertex is not well defined and there is, in essence, a continuum of verticity.

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Article type: Paper
DOI: 10.1039/DT9910003317
J. Chem. Soc., Dalton Trans., 1991, 3317-3324

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    When is a vertex not a vertex? An analysis of the structures of [MB10H12] metallaboranes

    S. A. Macgregor, A. J. Wynd, N. Moulden, R. O. Gould, P. Taylor, L. J. Yellowlees and A. J. Welch, J. Chem. Soc., Dalton Trans., 1991, 3317
    DOI: 10.1039/DT9910003317

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