The reaction of Cp(CO)2 Mo=CTol (Cp = eta-5-C5H5, Tol = p-C6H4Me) and (mu-H)2Os3(CO)10 under mild conditions (5-20-degrees-C) produces three molybdenum-triosmium mixed-metal cluster compounds, CpMoOS3(CO)11[mu-3-eta-2-C(O)CH2Tol] (4, 65%) as a major product together with two minor products CpMoOS3(CO)10(mu-3-CTol)2(mu-H) (5, 10%) and CpMoOs3(CO)10(mu-3-eta-2-C2Tol2)(mu-H) (6, 6%). Compounds 4, 5 and 6 have been isolated as crystalline solids and characterized by spectroscopic methods. Fluxionality of compound 4 has been examined by variable temperature C-13 NMR spectroscopy. Compound 5 in solution undergoes a degenerate metal framework rearrangement. H-1 NMR analysis of the tolyl methyl resonances of 5 for the rearrangement process gives DELTA-G(C)not-equal = 15.2 +/- 0.2 kcal/mol at the temperature of coalescence.