Reaction of {[2,5-Ph-2-3,4-Tol(2)(eta (5)-C4CO)](2)H}Ru-2(CO)(4)(mu -H) (6) with H-2 formed [2,5-Ph-2-3,4-Tol(2)(eta 5-C4COH)Ru(CO)(2)H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at -10 degreesC showed second-order kinetics with DeltaH(double dagger) = 12.0 kcal mol(-1) and DeltaS(double dagger) = -28 eu. The rate of reduction was not accelerated by CF3CO2H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 Save individual kinetic isotope effects k(RuH)/k(RuD) = 1.5 +/- 0.2 and k(OH)/k(OD) = 2.2 +/- 0.1 for PhCHO reduction at 0 degreesC. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of k(OHRuH)/k(ODRuD) = 3.6 +/- 0.3. [2,5-Ph-2-3,4-Tol(2)(eta (5)-C4COSiEt3)Ru(CO)(2)H] (12) and NEt4+[2,5-Ph-2-3,3-Tol(2)(eta (4)-C4CO)Ru(CO)(2)H](-) (13) were unreactive toward PhCHO under conditions where facile PhCHO reduction by 8 occurred. PhCOMe was reduced by 8 30 times slower than PhCHO; MeN=CHPh was reduced by 8 26 times faster than PhCHO. Cyclohexene was reduced to cyclohexane by 8 at 80 degreesC only in the presence of H-2. Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed.