The reduction of pentaammine(phenanthroline-2-carboxamido-N)cobalt(III) by chromium(II) proceeds by formation of a binuclear intermediate followed by intramolecular electron transfer. The observed rate constant can be expressed as k(obs) = K(p)k(et)[Cr2+]/(1 + K(p)[Cr2+]), where K(p) is the formation constant of the binuclear species and k(et) is the intramolecular electron-transfer rate constant within the binuclear intermediate. At 25-degrees-C, I = 1.0 M (LiClO4), K(p) = 28 M-1, DELTAH-degrees = 4.4 +/- 2 kcal mol-1, DELTAS-degrees = 22 +/-5 cal mol-1 deg-1, k(et) = 140 s-1, DELTAH(double dagger) = 2.6 +/- 0.4 kcal mol-1, and DELTAS(double dagger) = -40 +/- 6 cal mol-1 deg-1. Complete ligand transfer is observed, and the initial chromium(III) product is a chelate involving the carboxamido oxygen and one of the phenanthroline nitrogens. The subsequent reactions of this complex are also described.