The VCD method – a simple and reliable way to distinguish cage C and B atoms in (hetero)carborane structures determined crystallographically

Abstract

Replacing a boron vertex in a [B_nH_n]²⁻ cluster with a smaller atom, e.g. carbon, results in the distance from that atom to the polyhedral centroid being shorter. This forms the basis of a simple but very effective method of distinguishing between B and C atoms in (hetero)carboranes that have been studied crystallographically, the Vertex-to-Centroid Distance (VCD) method. Examples of well-characterised icosahedral and sub-icosahedral closocarboranes, nidocarboranes, icosahedral metallacarboranes and supraicosahedral metallacarboranes are used to illustrate the generality of the VCD method. In this process a number of structures are identified in which the method suggests B/C disorder not previously recognised and some structures in which it appears that a cage C atom has been wrongly assigned. The largest sub-group of heterocarboranes is the family of 3,1,2-MC₂B₉ compounds, and for these species consideration of Exopolyhedral Ligand Orientation (ELO) can sometimes be used to quickly suggest when a literature structure is suspect in terms of cage C atom positioning. The crystal structure of one such compound, 3,3-NO₃-3-PPh₃-3,1,2-closo-RhC₂B₉H₁₁, has been redetermined and the correct location of the cage C atoms established.