Treatment of the manganese acetyl (CO)(5)MnC(O)CH3 (1) with 1-2 equiv of a monohydrosilane furnished mixtures of alpha-siloxyethyl (CO)(5)MnCH(OSiR(3))CH3 (2) and alpha-siloxyvinyl (CO)(5)MnC(OSiR(3))=CH2 (3) complexes. Relative yields of 2 and 3 varied from 80% and 6% for HSiMe(2)Ph to 28% and 59% for HSiEt(3). One of the latter compounds, (CO)(5)MnC(OSiEt(3))=CH2 (3j), was fully characterized, and seven examples of 2 were isolated in moderate yields. Four alpha-siloxyethyl complexes 2 were further characterized as stable derivatives (CO)(5)MnC(O)CH(OSiR(3))CH3 after carbonylation. Mechanistic studies on the HSiMe(2)Ph and HSiEt(3) hydrosilation of 1 are noteworthy for (1) the absence of (R(3)SiO)CH=CH2 and CH(3)CH(2)OSiR(3) byproducts, (2) the presence of 3 but not (CO)(5)MnSiR(3), (3) inhibition by CO, phosphine, or acetonitrile, but neither air nor light, (4) competitive hydrosilation of other substrates (e.g., acetone or Cp(CO)(2)FeC(O)R) for which 1 is a precatalyst, (5) degradation of 2 by excess HSiR(3), giving Mn-2(CO)(10) and (R(3)Si)(2)O as the final products, (6) the fact that this degradation results in autocatalysis by generating the transient active catalyst (CO)(4)MnSiR(3) (15), and (7) the fact that the hydrosilation induction period can be removed by independently generating the putative 15. These observations are consistent with an autocatalytic hydrosilation mechanism in which silane degradation of product 2 (or of other manganese complexes) generates the active catalyst 15, which binds 1 and rearranges to the unsaturated bimetallic mu-siloxyethylidene (CO)(5)MnC(CH3)(OSiMe(2)Ph)Mn(CO)(4) as the key catalysis intermediate : silane addition affords 2 whereas beta-deinsertion produces 3.