Synthetic and kinetic studies are used to uncover mechanistic. details of the reduction of O-2 to water Mediated by dirhodium complexes. The mixed valence Rh-2(0,II)(tfepma)(2)((CNBu)-Bu-t)(2)Cl-2 (1, tfepma = MeN [P(OCH2CF3)(2)](2), (CNBu)-Bu-t = tert-butyl isocyanide) complex is protonated by HCl to produce Rh-2(II,II) (tfepma)(2)((CNBu)-Bu-t)(2)Cl3H (2), which promotes the reduction of 02 to water with concomitant of Rh-2(II,II)(tfepma)(2)((CNBu)-Bu-t)(2)Cl-4 (3). Reactions of the analogous diiridium complexes permit, the identification of plausible reaction intermediates. Ir-2(0,II)(tfepma)(2)((CNBu)-Bu-t)(2)Cl-2 (4) can be protonated to form the isolable complex Ir-2(II,II)(tfepma)(2)((CNBu)-Bu-t)(2)Cl3H (5), which reacts with O-2 to form Ir-2(II,II)(tfepma)(2)((CNBu)-Bu-t)(2)Cl-3(OOH) (6). In addition, 4 reacts with O-2 to form Ir-2(II,II)(tfepma)(2)((CNBu)-Bu-t)(2)Cl-2(eta(2)-I-2) (7), which can be protonated by HCl to furnish 6. Complexes 6 and 7 were both isolated in pure form and structurally and spectroscopically characterized. Kinetics examination of hydride complex 5 with O-2 and HCl furnishes a rate law that is consistent with an HCl-elimination mechanism, where O-2 binds an Ir-0 center to furnish an intermediate eta(2)-peroxide intermediate. Dirhodium,congener 2 obeys a rate law that not only is also consistent with an analogous HCl-elimination mechanism but also includes terms indicative of direct O-2 insertion. and a unimolecular isomerization prior to oxygenation. The combined synthetic and mechanistic hydride complexes.