We have reexamined the thermally induced Co(NH3)(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 find support for a previously suggested mechanism involving the excited state (E-2(g)) of Co(II) which becomes lower than the ground state (T-4(1g)) in the transition state region. This lowers the transition-state barrier considerably by similar to6.9 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 (T-3(1g)) of Co(III) and the fact that an optical absorption peak (E-2(g)) of the Co(II) species could not be found experimentally. Our rate is of order 6x10(-3) (M s)(-1) and hence two orders of magnitude faster than determined previously by experiments. (C) 2003 American Institute of Physics.