The electron self-exchange reaction FeCl(OH2)(5)(2+) + Fe(OH2)(6)(2+) -> Fe(OH2)(6)(2+) + FeCl(OH2)(5)(2+), proceeding via the inner-sphere pathway, was investigated with quantum chemical methods. Geometry and vibrational frequencies of the precursor/successor complex, (H2O)(5)(FeClFeII)-Cl-III(OH2)(5)(4+)/(H2O)(5)(FeClFeIII)-Cl-II(OH2)(5)(4+) (P/S), and the transition state, (H2O)(5)FeClFe(OH2)(5)(4+) (TS), were computed with the LC-BOP functional and CPCM hydration. Bent and linear structures were computed for the TS and P/S. The electronic coupling matrix element (H-ab) and the electronic energies were calculated with multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory (XGMC-QDPT2) and spin-orbit configuration interaction (SO-CI). Since the Fe Fe distance changes considerably along the electron transfer step, the transformation P -> TS -> S, equations based on the hypothesis of a fixed donor-acceptor distance cannot be applied. Hence, the rate constant for the electron transfer step (k(et)) was calculated as described previously (Rotzinger, F. P. Inorg. Chem. 2014, 53, 9923). k(et) is very fast, similar to 9.4 x 10(8-)6.6 x 10(9) s(-1) at 0 degrees C. The experimental rate constant of the title reaction (k) is much slower and controlled by the formation of the precursor complex. The substitution of a water ligand by FeCl(OH2)(5)(2+) at Fe(OH2)(6)(2+) is rate-determining.