Reduction of 1,2-closo-C2B10H12 followed by treatment with [RuCl2(p-cymene)]2 (p-cymene = C 6H4MeiPr-1,4) affords the 13-vertex ruthenacarborane 4-(p-cymene)-4,1,6-closo-RuC2B10H 12, characterised both spectroscopically and, in two crystalline forms, crystallographically. Although asymmetric in the solid state, having a docosahedral cage architecture with cage C atoms at vertices 1 and 6, this species clearly has Cs symmetry on the NMR timescale at room temperature. However, the fluctional process in operation can be arrested at low temperature, and an activation energy of 43.1 kJ mol-1 is estimated. A computational study of the related species 4-(?-C6H 6)-4,1,6-closo-RuC2B10H12 reveals that the fluctionality is due to a double diamond-square-diamond process, first suggested by Hawthorne et al for the analogous CpCo species. These calculations yield an activation energy of 40.4 kJ mol-1, in excellent agreement with that derived from experiment. Reduction of 1,2-Ph2-1,2-closo- C2B10H10 followed by treatment with [RuCl 2(?-C6H6)]2 or [RuCl 2(p-cymene)]2 yields the analogous species 1,6-Ph 2-4-(?-C6H6)-4,1,6-closo-RuC 2B10H10 and 1,6-Ph2-4-(p-cymene)-4, 1,6-closo-RuC2B10H10, respectively. These C,C-diphenyl compounds were again studied spectroscopically and crystallographically, the p-cymene species again showing two crystalline modifications. In contrast to their CpCo and Cp*Co analogues all three ruthenacarboranes do not undergo isomerisation in refluxing toluene. © The Royal Society of Chemistry 2005.