Reductions using In-1(aq) and Ti-III(aq), of (NH3)(5)Ru-III derivatives of pyridines having carbonyl-bearing substituents (-CONH2, -COOCH3, and -COC6H5) yield the corresponding (NH3)(5)Ru-II complexes. Reactions with Ti-III are kinetically straightforward, exhibit only slight responses to structural alteration, and give no indication of inner-sphere mediation involving the carbonyl group. Kinetic profiles for In(I) reductions of the 3-CONH2, 3-COOCH3, and 3-COC6H5-substituted complexes (in the range [H+] = 0.030-0.15 M) begin with a nearly linear section, with rates independent of [In-1], but show curvature during the later stages of reaction. These profiles are consistent with a reaction sequence in which the predominant carbonyl form of the oxidant is hydrated to a more reactive gem-diol form (>C=O + H2O reversible arrow C(OH)(2)), which undergoes reduction by In-1, yielding the observed Ru-II product and In-II. The latter is then rapidly consumed by a second unit of Ru-III. Rate constants for the hydration step giving optimal fit to the experimental curves are 6 x 10(-4) s(-1) (for the 3-CONH2 complex) and 1.3 x 10(-3) s(-1) (for the 3-COOCH3 oxidant). Lower limits for the rate of attack by In-1 on the active forms of the oxidants are estimated as 10(3.3)-10(4.6) M-1 s(-1), about 10(2)-10(3) times are rapid as the reduction of the unsubstituted pyridine complex. Our results suggest the utilization, by In-1, of a hydroxyl-bridged path featuring the sequence In-1-OH-C(OH)-py in these reductions, a path which is overshadowed, in the case Ti-III reactions, by a more failce outersphere process.