Reduction of the tethered carborane 1,2-(CH2) 3-1,2-closo-C2B10H10 followed by treatment with CoCl2/NaCp, [(p-cymene)RuCl2]2 (p-cymene = C6H4MeiPr-1,4), (PMe 2Ph)2PtCl2 or (dppe)NiCl2 (dppe = Ph2PCH2CH2PPh2) affords reasonable yields of the new 13-vertex metallacarboranes 1,2-(CH2) 3-4-Cp-4,1,2-closo-CoC2B10H10 (1), 1,2-(CH2)3-4-(p-cymene)-4)1,2-closo-RuC2B 10H10 (2), 1,2-(CH2)3-4,4-(PMe 2Ph)2-4,1,2-closo-PtC2B10H 10 (3) and 1,2-(CH2)3-4,4-(dppe)-4,1,2-closo- NiC2B10H10 (4), respectively. All compounds were characterised spectroscopically and crystallographically. The cobalt and ruthenium species 1 and 2 have CS symmetry in both solution and the solid state, having henicosahedral cage structures featuring a trapezoidal C1C2B9B5 face. The platinum and nickel compounds 3 and 4 have asymmetric docosahedral cage structures in the crystal (the more so for 4 than for 3) although both appear, by 11B and 31P NMR spectroscopy, to have CS symmetry in solution. Low-temperature experiments on the more soluble platinacarborane could not freeze out the diamond-trapezium-diamond fluctional process that we assume is operating in solution, and we therefore conclude that this process has a relatively low activation barrier, probably <35 kJ mol-1. © The Royal Society of Chemistry 2005.