The title reaction was theoretically investigated, where cis-[RhH2(PH3)(3)](+) and cis-[RhH2(PH3)(2)(H2O)](+) were adopted as models of the catalyst. The first step of the catalytic cycle is the CO2 insertion into the Rh(III)-H bond, of which the activation barrier (E-a) is 47.2 and 28.4 kcal/mol in cis-[RhH2(PH3)(3)](+) and cis-[RhH2(PH3)(2)(H2O)](+), respectively, where D FT(B3LYP)-calculated E. values (kcal/mol unit) are given hereafter. These results indicate that an active species is not cis-[RhH2(PH3)(3)](+) but cis-[RhH2(PH3)(2)(H2O)](+), After the CO2 insertion, two reaction courses are possible. In one course, the reaction proceeds through isomerization (E-a = 2.8) of [RhH(eta(1)-OCOH)(PH3)(2)(H2O)(2)](+), five-centered H-OCOH reductive elimination (E-a = 2.7), and oxidative addition of H-2 to [Rh(PH3)(2)(H2O)(2)](+) (E-a = 5.8). In the other one, the reaction proceeds through isomerization of [RhH(eta(1)-OCOH)(PH3)(2)(H2O)(H-2)](+) (E-a = 5.9) and six-centered sigma-bond metathesis of [RhH(eta(1)-OCOH)(PH3)(2)(H2O)](+) with H-2 (no barrier). RhH(PH3)(2)-catalyzed hydrogenation of CO2 proceeds through CO2 insertion (E-a = 1.6) and either the isomerization of Rh(eta(1)-OCOH)(PH3)(2)(H-2) (E = 6,11) followed by the six-centered a-bond metathesis (E-a = 0.3) or H-2 oxidative addition to Rh(eta(1)-OCOH)(PH3)(2) (E-a = 7.3) followed by isomerization of RhH2(eta(1)-OCOH)(PH3)(2) (E-a = 6.2) and the five-centered H-OCOH reductive elimination (E-a = 1.9). From these results and our previous results of RuH2(PH3)(4)-catalyzed hydrogenation of CO2 (J. Am. Chem. Soc. 2000, 122, 3867), detailed discussion is presented concerning differences among Rh(III), Rh(I), and Ru(II) complexes.