Hydride transfer from dihydronicotinamide adenine dinuclectide (NADH) analogs to a manganese(IV)-oxo porphyrin complex, (TMP)Mn-IV(O) [TMP = 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphyrin], occurs via disproportionation of (TMP)Mn-IV(O) to [(TMP)Mn-III](+) and [(TMP)Mn-V(O)](+) that acts as the actual hydride acceptor. In contrast, electron transfer from ferrocene derivatives to (TMP)Mn-IV(O) occurs directly to afford ferricenium ions and (TMP)Mn-III(OH) products. The disproportionation rate constant of (TMP)Mn-IV(O) was determined by the dependence of the observed second-order rate constants on concentrations of NADH analogs to be (8.0 +/- 0.6) x 10(6) M-1 s(-1) in acetonitrile at 298 K. The disproportionation rate constant of (TMP)Mn-IV(O) in hydride-transfer reactions increases linearly with increasing acid concentration, whereas the rate constant of electron transfer from ferrocene to (TMP)Mn-IV(O) remains constant irrespective of the acid concentration. The rate constants of electron transfer from a series of ferrocene derivatives to (TMP)Mn-IV(O) were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energy of electron transfer by the (TMP)Mn-IV(O) complex.