This report describes the thermochemistry, proton-coupled electron transfer (PCET) reactions and self-exchange rate constants for a set of bis-benzimidazolate-ligated [2Fe-2S] clusters. These clusters serve as a model for the chemistry of biological Rieske and mitoNEET clusters. PCET from [Fe2S2((Pr)bbim)((Pr)bbimH)](2-) (4) and [Fe2S2((Pr)bbim)((Pr)bbimH(2))](1-) (5) to TEMPO occurs via concerted proton-electron transfer (CPET) mechanisms ((Pr)bbimH(2) = 4,4-bis-(benzimidazol-2-yl)heptane). Intermolecular electron transfer (ET) self-exchange between [Fe2S2((Pr)bbim)(2)](2-) (1) and [Fe2S2((Pr)bbim)(2)](3-) (2) occurs with a rate constant of (1.20 +/- 0.06) x 10(5) M-1 s(-1) at 26 degrees C. A similar self-exchange rate constant is found for the related [2Fe-2S] cluster [Fe2S2(SArO)(2)](2-/3-), SArO2- = thiosalicylate. These are roughly an order of magnitude slower than that reported for larger [4Fe-4S] clusters and 1 order of magnitude faster than that reported for N-ligated high-spin iron complexes. These results suggest that the rate of intermolecular ET to/from [Fe-S] clusters is modulated by cluster size. The measured PCET self-exchange rate constant for 1 and 4 at -30 degrees C is (3.8 +/- 0.7) x 10(4) M-1 s(-1). Analysis of rate constants using the Marcus cross-relation suggests that this process likely occurs via a concerted proton-electron transfer (CPET) mechanism. The implications of these findings to biological systems are also discussed, including the conclusion that histidine-ligated [2Fe-2S] clusters should not have a strong bias to undergo concerted e(-)/H+ transfers.