Room temperature reactions of the rhodium and iridium bromides (η-9-SMe2-7,8-Me2-7,8-C2B9H8)MBr2 (M = Rh (1a), Ir (1b)) with CpTl give the dicarbollide thioether complexes (η-9-SMe-7,8-Me2-7,8-C2B9H8)MCp (2a,b), as a result of demethylation of the SMe2 substituent. These reactions are accompanied by transformation of cage structure from pseudocloso to closo. Refluxing of the sulfonium derivatives [(η-9-SMe2-7,8-Me2-7,8-C2B9H8)MCp]+ (3a,b) with [Bu4N]Br also affords 2a,b. In the same manner ruthenacarboranes [(η-9-SMe2-7,8-C2B9H10)Ru(η-C6H6)]+ (4a) and [(η-10-SMe2-7,8-C2B9H10)Ru(η-C6H6)]+ (4b) react with halide ions giving the thioether complexes (η-9-SMe-7,8-C2B9H10)Ru(η-C6H6) (5a) and (η-10-SMe-7,8-C2B9H10)Ru(η-C6H6) (5b). Dependence of the demethylation rate on the nature of halide ion and solvent was studied both experimentally and theoretically. The structures of 2a and 3aPF6 were determined by X-ray diffraction.