Bonding and electron counting in hyper-closo metalloboranes and metallocarbaboranes

Since the characterisation of the first hyper-or iso-closo metalloboranes and metallocarbaboranes there has been considerable debate regarding the most appropriate formal electron counting scheme for these species. While they clearly show deviations from the usual geometries as a result of a formal electron deficiency, there is some disagreement as to whether the electron deficiency should be considered on the metal centre (iso-closo) or associated with the delocalised cluster bonding (hyper-closo). We review the structural evidence and some of the theoretical contributions to the debate, and describe MO calculations which address the validity of the alternative electron counting schemes. Although we find that the hyper-closo model is more generally applicable, the structures show such a diversity of bonding characteristics that any simple attempt to generalise electron counting rules is unlikely to be applicable to all known examples. 53 refs