The kinetics of the comproportionation reaction of Ru(tpy)(bpy)O2+ and Ru(tpy)(bpy)OH22+ to produce Ru(tpy)(bpy)OH2+ were evaluated in a series of complexes that were substituted on the 4' position of the tpy ligand or the 4 and 4' positions of the bpy ligand (tpy = 2,2':6',2 "-terpyridine, bpy = 2,2'-bipyridine). These substitutions did not change the steric or coordination environments about the Ru-O linkage but did modulate the driving force (-Delta G degrees) for comproportionation over a range of similar to 8 kJ/mol. The comproportionation reaction, which involves a net hydrogen atom transfer between the metal complexes, showed a linear dependence of its rate constant on the driving force across the range studied, with a slope of 0.66 +/- 0.06 for H2O and 0.64 +/- 0.05 for D2O. Thus, the slopes were in reasonably goad agreement with the value of 0.5 predicted by Marcus theory and, as also expected, showed no effect of the driving force an the isotope effect. The isotope effect for the Ru(tpy)(bpy)O2+ complex (11.5) was significantly lower than that for Ru(bpy)(2)(py)O2+ (16.1) at the same driving force. The Ru(bpy)(2)(py)O2+ complex is more sterically crowded at the oxo ligand, so the likely origin of the isotope effect is the distance of transfer for the proton in the reaction.