The electron self-exchange rate for the couple [NiIV/IIIMe2L(l)]2+/+(Me2L(1)H2= 3,14-dimethyl-4,7,10,13-tetraazahexadeca-3,13-diene-2,15-dione dioxime) has been determined as 2.4 × 104 M−1s−1at 25 °C and 0.10 M ionic strength by 1H NMR line broadening. Rate constants for cross-reactions calculated with use of this self-exchange rate in the Marcus relationship show good agreement with measured values. The effect of the paramagnetic ions [Cr(edta)]−, [Niiii, Me2L(1)]+, and [Cr(phen)3]3+on the ’H NMR relaxation rates of the cobalt(III) complex [ComMe2L(1)]+ has been used to investigate ion-pair structure in solution. These ion pairs serve as models for the electron-transfer precursor assemblies in the self-exchange reaction and two cross-reactions, respectively. Marcus calculations are relatively insensitive to the precursor structures. However, these structures provide a basis for interpretation of stereoselectivity data.