Bimolecular electron transfer quenching of the (MLCT)-M-3 excited state of [Ru(dmb)(3)](2+) by dinuclear complexes of Fe-III has been observed. Compounds of the general form [Fe-2(mu-O(H))(mu-O2CCH3)(2)(Tp)(2)](n+) (Tp = hydrotris(1-pyrazolyl)borate) react following photoexcitation of [Ru(dmb)(3)](2+) with bimolecular quenching rate constants of k(q) = (7.1 +/- 0.3) x 10(8) M-1 s(-1) for Fe2O(O2CCH3)(2)(Tp)(2) and (5.15 +/- 0.15) x 10(9) M-1 s(-1) for [Fe-2(OH)(O2CCH3)(2)(Tp)(2)](+). It is suggested that the inferred difference in electron transfer rates may be related to the change in intramolecular Heisenberg exchange that accompanies protonation of the oxo bridge.