Density functional theory calculations with the M06 functional have been performed on the reactivity, selectivity and mechanisms of hydrosilylations of alkynes, ketones and nitriles catalyzed by cationic ruthenium complexes [CpRu(L)(MeCN)2]+ with L = PiPr3 or MeCN. The hydrosilylation of alkynes with L = PiPr3 involve an initial silyl migration mechanism to generate the anti-Markovnikov product, in contrast to the Markovnikov product obtained with L = MeCN. The bulky phosphine ligand directs the silyl group to migrate to Cbeta of the alkyne. This explains the anti-Markovnikov selectivity of the catalyst with L = PiPr3. By contrast, the silane additions to either ketone or nitrile proceed through an ionic SN2-Si outer sphere mechanism, in which the substrate attacks the Si center. The PiPr3 ligand facilitates the activation of the Si-H bond to furnish a eta2-silane complex, while a eta1-silane complex is formed for the MeCN ligand. This property of the phosphine ligand enables the catalytic hydrosilylation of ketones and nitriles in addition to alkynes.