Bridges and Vertices in Heteroboranes.

A number of (hetero)boranes are known in which a main group atom X 'bridges' a B-B connectivity in the open face, and in such species X has previously been described as simply a bridge or, alternatively, as a vertex in a larger cluster. In this study we describe an approach to distinguish between these options based on identifying the best fit of the experimental {B x } cluster fragment with alternate exemplar {B x } fragments derived from DFT-optimized [B n H n ] 2- models. In most of the examples studied atom X is found to be better regarded as a vertex, having 'a 'verticity' of ca. 60-65%. Consideration of our results leads to the suggestion that the radial electron contribution from X to the overall skeletal electron count is more significant than the tangential contribution.