Reported here are self-exchange reactions between iron 2,2'-bi(tetrahydro)pyrimidine (H(2)bip) complexes and between cobalt 2,2'-biimidazoline (H(2)bim) complexes. The H-1 NMR resonances of [Fe-II(H(2)bip)(3)](2+) are broadened upon addition of [Fe-III(H(2)bip)(3)](3+), indicating that electron self-exchange occurs with k(Fe,e-) = (1.1 +/- 0.2) x 10(5) M-1 s(-1) at 298 K in CD3CN. Similar studies of (Fe-II(H(2)bip)(3)](2+) plus [Fe-III(Hbip)(H(2)biP)(2)](2+) indicate that hydrogen-atom self-exchange (proton-coupled electron transfer) occurs with k(Fe,H.) = (1.1 +/- 0.2) x 10(4) M-1 s(-1) under the same conditions. Both self-exchange reactions are faster at lower temperatures, showing small negative enthalpies of activation: DeltaH(double dagger)(e(-)) = -2.1 +/- 0.5 kcal mol(-1) (288-320 K) and DeltaH(double dagger)(H-.) = -1.5 +/- 0.5 kcal mol(-1) (260-300 K). This behavior is concluded to be due to the faster reaction of the low-spin states of the iron complexes, which are depopulated as the temperature is raised. Below about 290 K, rate constants for electron self-exchange show the more normal decrease with temperature. There is a modest kinetic isotope effect on H-atom self-exchange of 1.6 +/- 0.5 at 298 K that is close to that seen previously for the fully high-spin iron biimidazoline complexes.(12) The difference in the measured activation parameters, E-a(D) - E-a(H), is -1.2 +/-0.8 kcal mol(-1), appears to be inconsistent with a semiclassical view of the isotope effect, and suggests extensive tunneling. Reactions of [Co(H(2)biM)(3)](2+)-d(24) with [Co(H(2)bim)(3)](3+) or [Co(Hbim)(H(2)bim)(2)](2+) occur with scrambling of ligands indicating inner-sphere processes. The self-exchange rate constant for outer-sphere electron transfer between [Co(H(2)bim)(3)](2+) and [Co(H(2)bim)(3)](3+) is estimated to be 10(-6) M-1 s(-1) by application of the Marcus cross relation. Similar application of the cross relation to H-atom transfer reactions indicates that self-exchange between [Co(H(2)bim)(3)](2+) and [Co(Hbim)(H(2)bim)(2)](2+) is also slow, less than or equal to10(-3) M-1 s(-1). The slow self-exchange rates for the cobalt Complexes are apparently due to their interconverting high-spin [Co-II(H(2)bim)(3)](2+) with low-spin Co(Ill) derivatives.