The mechanism of peroxide dismutation by synthetic mimics of the dimanganese catalase enzymes has been investigated by steady-state kinetic methods. These compounds, [LMn(2)(II,II)(mu-X)](ClO4)(2), X(-) = CH3CO2- and ClCH2CO2-, were found to share structural, redox, and spectroscopic properties analogous to the catalase enzymes (Pessiki et al. J. Am. Chem. Sec. preceding paper in this issue). The dismutation mechanism proceeds by two consecutive two-electron steps : H2O2 + 2e(-) + 2H(+) --> 2H(2)O and H2O2 --> O-2 + 2e(-) + 2H(+) which are coupled to redox transformation of the catalyst : Mn-2(III),(III) <----> Mn-2(II),(II). The mu-carboxylate derivatives are inactive, but in the presence of water they autocatalytically dismutate H2O2 after an initial hydration reaction in which the mu-carboxylate ligand appears to dissociate, as judged by inhibition with acetate. The observed steady-state rate expression, nu(O-2) = k(obs)[H2O2](1)- [(LMn(2)(CH3CO2)(ClO4)(2)],(1) k(obs) = 0.23 M(-1) s(-1), exhibits the same molecularities with respect to peroxide and catalyst as observed for the enzyme from T. thermophilus, for which k(obs) is 10(7) faster. In contrast, the rate law for the mu-Cl-derivative, LMn(2)Cl(3), is second order in [H2O2] (Mathur et al. J. Am. Chem. Soc. 1987, 109, 5227). EPR and optical studies support a mechanism involving oxidation to a Mn-2(III,III) intermediate and against formation of the mixed valence states, Mn-2(II,III) and Mn-2(III,IV). The rate-limiting step for the model complexes is ascribed to either the inner-sphere two-electron intramolecular oxidation of the peroxide complex, [LMn(2)(II,II)(H2O2)](3+) --> [LMn(2)(III,III)(OH)(2)](3+), or a proton dissociation reaction coupled to this oxidation. Subsequent two-electron reduction to the Mn-2(II,II) oxidation state via a second H2O2 molecule occurs 7-9-fold faster and completes the catalytic cycle. The 10(7) faster rate for the enzyme is proposed to reflect either a substantially lower reduction potential for the MnCat(III,III) oxidation state, the availability of active site residues which function as proton donors and acceptors, or both.