The VCD Method--a Simple and Reliable Way to Distinguish Cage C and B Atoms in (hetero)carborane Structures Determined Crystallographically

Replacing a boron vertex in a [B(n)H(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 closo carboranes, nido carboranes, 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-MC(2)B(9) 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-NO(3)-3-PPh(3)-3,1,2-closo-RhC(2)B(9)H(11), has been redetermined and the correct location of the cage C atoms established.