The activation parameters reported for intramolecular electron-transfer between ruthenium(II) and cobalt(III) complexes have been corrected for the thermodynamic contributions of the entropy change for the reaction Delta S-0 to the entropy of activation Delta S*, and it is concluded that these electron-transfers range from adiabatic (pyrazine bridge) to highly nonadiabatic. The electronic factors are about 20 times smaller than for diruthenium mixed-valence complexes. Spanning the metal-metal separation range of 7-14 angstrom over which beta = 0.7 angstrom(-1), the electron-transfer rates are dominated by the electronic factors, which change three times more with separation than do the nuclear factors.