Theory of adiabatic Hexaamminecobalt-Self-Exchange

We have reexamined the thermally induced Co(NH_3)_6^{2+/3+} [Co(II/III)] redox reaction using the first principles density-functional-theory method, semiclassical Marcus theory, and known charge transfer parameters. We confirm a previously suggested mechanism involving excited state (^2E_g) of Co(II) which becomes lower than the ground state (^4T_1g) in the transition state region. This lowers the transition state barrier considerably by about 6.9 kcal/mol and leads to a spin-allowed and adiabatic electron exchange process. Our calculations are consistent with previous experimental results regarding the spin-excitation energy (^3T_1g) of Co(III), and the fact that an optical absorption peak (^2E_g) of the Co(II) species could not be found experimentally. Our rate is of order 6 10^{-3} 1/Ms and hence 2 orders of magnitude faster than determined previously by experiments.