Mechanistic implications of proton transfer coupled to electron transfer

The kinetics of electron transfer for the reactions cis- [Ru-IV(bpy)(2)(PY)(O)](2+) + H+ + [Os-II(bpy)(3)](2+) reversible arrow cis- [Ru-III(bpy)(2)(py)(OH)](2+) + [Os-III(bpy)(3)](3+) and cis- [Ru-III(bpy)(2)(py)(OH)](2+) + H+ + [Os-II(bpy)(3)](2+) cis- [Ru-II(bpy)(2)(py) (H2O)](2+) + [Os-III(bpy)(3)](3+) have been studied in both directions by varying the pH from 1 to 8. The kinetics are complex but can be fit to a double "square scheme" involving stepwise electron and proton transfer by including the disproportionation equilibrium, 2cis- [Ru-III(bpy)(2)(py)(OH)](2+) reversible arrow (3 x 10(3) M-1 s(-1) forward, 2.1 x 10(5) M-1 s(-1) reverse) cis [Ru-IV(bpy)(2)(py)(O)](2+) + cis-[Ru-II(bpy)(2)(py)(H2O)](2+) Electron transfer is outer-sphere and uncoupled from proton transfer. The kinetic study has revealed (1) off-dependent reactions where the pH dependence arises from the distribution between acid and base forms and not from variations in the driving force; (2) competing pathways involving initial electron transfer or initial proton transfer whose relative importance depends on pH; (3) a significant inhibition to outer-sphere electron transfer for the Ru-IV=O-2+/Ru-III-OH2+ couple because of the large difference in pK(a) values between Ru-IV=OH3+ (pK(a)<0) and (RuOH2+)-O-III (pK(a)>14); and (4) regions where proton loss from cis- [Ru-II(bpy)(2)(py)(H2O)](2+) or cis- [Ru-III(bpy)(2)(py)(OH)](2+) is rate limiting. The difference in pK(a) values favors more complex pathways such as proton-coupled electron transfer.