The rate constant, k, for the homogeneous electron transfer (self-exchange) reaction between the diamagnetic bis(maleonitriledithiolato)nickel dianion, (Ni(mnt)(2)](2-), and the paramagnetic monoanion, [Ni(mnt)(2)](1-), has been determined in acetone and nitromethane (CH3NO2) using C-13 NMR line widths at 22 degrees C (mnt = 1,2-S2C2(CN)(2)). The values of k (2.91 x 10(6) M-1 s(-1) in acetone, 5.78 x 10(6) M-1 s(-1) in CH3NO2) are faster than those for the electron transfer reactions of other Ni(III,II) couples; the structures of (Ni(mnt)(2)](1-) and [Ni(mnt)(2)](2-) allow for a favorable overlap that lowers the free energy of activation. The values of k are consistent with the predictions of Marcus theory. In addition to k, the spin-lattice relaxation time, T-1e, of [Ni(mnt)(2)](1-) is obtained from the NMR line width analysis; the values are consistent with those predicted by spin relaxation theory.