The hydrosilylation of unsaturated carbonheteroatom (C=O, C=N) bonds catalyzed by high-valent rhenium(V)dioxo complex ReO2I(PPh3)(2) (1) were studied computationally to determine the underlying mechanism. Our calculations revealed that the ionic outer-sphere pathway in which the organic substrate attacks the Si center in an eta(1)-silane rhenium adduct to prompt the heterolytic cleavage of the SiH bond is the most energetically favorable process for rhenium(V)dioxo complex 1 catalyzed hydrosilylation of imines. The activation energy of the turnover-limiting step was calculated to be 22.8 kcal/mol with phenylmethanimine. This value is energetically more favorable than the [2 + 2] addition pathway by as much as 10.0 kcal/mol. Moreover, the ionic outer-sphere pathway competes with the [2 + 2] addition mechanism for rhenium(V)dioxo complex 1 catalyzing the hydrosilylation of carbonyl compounds. Furthermore, the electron-donating group on the organic substrates would induce a better activity favoring the ionic outer-sphere mechanistic pathway. These findings highlight the unique features of high-valent transition-metal complexes as Lewis acids in activating the SiH bond and catalyzing the reduction reactions.