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

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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-Huckel 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.