A room-temperature reaction between the [7-(BuNH)-Bu-t-nido-7,8,9-C3B8H10](-) anion (1a) and [Cp*RUCl](4) leads to the ruthenatricarbollide [1-Cp*-12-(BuNH)-Bu-t-1,2,4,12-RUC3B8H10] (2) (yield 85%). Analogously, the room-temperature photochemical reaction of la with [CpFe(C6H6)]PF6 gives the previously reported iron complex [1-Cp-12-(BuNH)-Bu-t-1,2,4,12-FeC3B8H10] (3) (yield 82%). Both reactions are associated with extensive polyhedral rearrangement, which occurs under very mild conditions and brings the carbon atoms to positions of maximum separation within the framework. Compounds 2 and 3 were also surprisingly obtained via complexation of the isomeric [8-(BuNH)-Bu-t-nido7,8,9-C3B8H10](-) (1b) anion. Complex 2 rearranges further to [1-Cp*-10-(BuNH)-Bu-t-1,2,4,10-RuC3B8H10] (4) upon refluxing in xylene (145 degrees C). Density functional theory calculations at the B3LYP/SDD level were used to estimate relative stabilities of these metallacarborane isomers. Compounds 2 and 4, along with the 11-vertex closo compounds [1-Cp*-1,2,3,10-RUC3B7H10] (5) and [1-Cp*-10-'BuNH-1,2,3,10-RUC3B7H9] (6), were also isolated from the reaction between [Cp*RuCl2](2) and la in boiling xylene. The structure of 2 was established by an X-ray diffraction study, and the constitution of all compounds was determined unambiguously by multinuclear NMR spectroscopy, mass spectrometry, and elemental analyses.