We hypothesize that the more rapid rates for hydride transfer observed for NADH over formate reflects a kinetic rather than thermodynamic preference, given that formate (∆Gº=24.1 kcal/mol in H2O) has more favorable endergonicity than NAD(P)H (∆Gº=28.9 kcal/mol in H2O) for hydride transfer. We speculate that this kinetic difference may be due to m-amide coordination of the dihydropyridine moiety of NADH to the iridium center prior to hydride transfer via a 6-membered transition state, whereas formate hydride transfer would have a more entropically challenging 4-membered, non-coordinating