  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.
