The synthesis and characterization of four isomers of the mixed-metal bimetallocarborane C5H52CoNiCB7H8 are reported. These species have been found to undergo novel thermal polyhedral rearrangements werein metal atoms migrate to adjacent polyhedral vertices and remain there, constituting the first example of a thermally stable metal-metal interaction in a metallocarborane. Also described are some brominated derivatives and their thermal rearrangements. A general rearrangement scheme for ten-vertex polyhedra is developed and used in the analysis of polyhedral rearrangements and assignment of structures. In an attempt to synthesize a larger homologue of this mixed-metal species the authors isolated eta-C10H8CoIIICB10H11, the first metallocarborane containing a neutral arene ligand bound to the metal.