Metal-to-metal silyl migration and silicon-carbon bond cleavage/re-formation processes in the methylene silyl complexes Cp*Ru-2(2)(mu-CH2)(SiR3)(mu-Cl)

Ruthenium methylene/silyl complexes of stoichiometry Cp*Ru-2(2)(mu-CH2)(SiR 3)(mu-Cl), where SiR3 = SiMe3 (1), SiEt3 (2), SiMe2Et (3), and SiMe2Ph (4), are produced when [Cp*RuCl](4) is treated with the appropriate dialkylmagn esium reagent, Mg(CH2SiR3)(2). Each complex undergoes two fluxional process es as observed by variable-temperature H-1 NMR spectroscopy. The low-temper ature exchange process is migration of the SiR3 unit from one Ru center to the other, whereas the high-temperature process is the reversible re-format ion of the C-Si bond between the silyl group and the bridging methylene uni t. The activation parameters for the low-temperature exchange process in 1- 4 are sensitive to the nature of the silyl group: Delta K-double dagger bec omes smaller and Delta S-double dagger becomes more negative if the SiR3 gr oup bears nonidentical or larger, more flexible substituents. This finding suggests that the transition state for this exchange process is crowded or characterized by different amounts of solvent reorganization depending on t he SiR3 group involved. In contrast, the activation parameters for the high -temperature process in 1-4 are relatively independent of the nature of the silyl ligand. The small variation in the activation parameters for reforma tion of the C-Si bond is consistent with a noncrowded transition state in w hich the solvent reorganization is relatively independent of the nature of the SiR3 group.