THIOSULFATE-CATALYZED REDUCTION OF TRIS(ACETYLACETONATO)-MANGANESE(III) AND ITS HYDROLYTIC DERIVATIVES IN AQUEOUS PERCHLORATE-MEDIA

The kinetics of reactions of S2O3(2-) with [Mn(acac)3] (Hacac = acetylacetone) and its derivatives [Mn(acac)2(H2O)2]+ and [mn(acac)2(H2O)(OH)] have been investigated in aqueous perchlorate media containing different total quantities of acetylacetone (C(acac)) over the range pH 4.2-8.5. The observed dependences of the reaction rates on pH and C(acac) arise from hydrolytic equilibria of [Mn(acac)3]. The reaction is first order in [S2O3(2-)] but consumes no S2O3(2-). Instead, 1 mol of acetylacetone is consumed per mol of Mn(III) reduced. EPR and kinetic data suggest a S2O3(2-)/S2O3- catalytic cyclic which provides a low-energy path for the acac --> Mn(III) electron transfer. Rate constants for reduction of [Mn(acac)3] (k1), [MN(H2O2]+ (k2) and [Mn(acac)2(H2O)(OH)] (k3) follow the sequence k1 < k2 almost-equal-to k3. An inner-sphere mechanism plausible for the k2 and the k3 paths; k1 prpbably an outer-sphere process.