The electrophilic eta(3)-H2SiRR' sigma-complexes [PhBP3Ph]RuH(eta(3)-H2SiRR'] (RR' = MePh, 1a; Ph2, lb; [PhBP3Ph](-) = [PhB(CH2PPh2)(3)](-)) are efficient catalysts (0.01-2.5 mol % loading) for the hydrosilation of ketones with PhMeSiH2, Ph2SiH2, or EtMe2SiH. An allroxy complex [PhBP3Ph]Ru-OCHPh2 (4b) was observed (by P-31{H-1} NMR spectroscopy) as the catalyst resting state during hydrosilation of benzophenone with EtMe2SiH. A different catalyst resting state was observed for reactions using PhMeSiH2 or Ph2SiH2, and was identified as a silane sigma-complex [PhBP3Ph]RuH[eta(3)-H-SiRR'(OCHPh2)] (RR' = MePh, 5a; Ph-2, 5b) using variable temperature multinuclear NMR spectroscopy (-80 to 20 degrees C). The hydrosilation of benzophenone with PhMeSiH2 and 1a was examined by H-1 NMR spectroscopy at -18 degrees C (in CD2Cl2), and this revealed that either 1a, 5a, or both 1a and 5a could be observed as resting states of the catalytic cycle, depending on the initial [PhMeSiH2]:[benzophenone] ratio. Kinetic studies revealed two possible expressions for the rate of product formation, depending on which catalyst resting state was present (rate = k(obs)[PhMeSiH2][5a] and rate = k'(obs)[benzophenone][1a]) Computational methods (DFT, b3pw91, 6-31G(d,p)/LANL2DZ) were used to determine a model catalytic cycle for the hydrosilation of acetone with PhMeSiH2. A key step in this mechanism involves coordination of acetone to the silicon center of 1a-DFT, which leads to insertion of the carbonyl group into an Si-H bond (that is part of a Ru-H-Si 3c-2e bond). This generates an intermediate analogous to Sa (5a-i-DFT), and the final product is displaced from 5a-i-DFT by an associative process involving PhMeSiH2.