The cationic ruthenium silylene complex [Cp*(iPr3P)Ru(H)2(SiHMes)][CB11H6Br6], a catalyst for olefin hydrosilations with primary silanes, was isolated and characterized by X-ray crystallography. Relatively strong interactions between the silylene Si atom and Ru–H hydride ligands appear to reflect a highly electrophilic silicon center. The mechanism of olefin hydrosilation was examined by kinetics measurements and other experiments to provide the first experimentally determined mechanism for the catalytic cycle. This mechanism involves a fast, initial addition of the Si–H bond of the silylene complex to the olefin. Subsequent elimination of the product silane produces an unsaturated intermediate, which can be reversibly trapped by olefin or intercepted by the silane substrate. The latter reaction pathway involves activation of the reactant silane by Si–H oxidative addition and α-hydrogen migration to regenerate the key silylene intermediate.
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