Electron Transfer in Uranyl(VI)−Uranyl(V) Complexes in Solution

The rates and mechanisms of the electron self-exchange between U(V) and U(VI) in solution have been studied with quantum chemical methods. Both outer-sphere and inner-sphere mechanisms have been investigated; the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. The calculated rate constant for the self-exchange reaction UO2 +(aq) + UO2 2+(aq) ⇔ UO2 2+(aq) + UO2 +(aq), at 25 °C, is k = 26 M-1 s-1. The lower limit of the rate of electron transfer in the inner-sphere complexes is estimated to be in the range 2 × 104 to 4 × 106 M-1 s-1, indicating that the rate for the overall exchange reaction may be determined by the rate of formation and dissociation of the binuclear complex. The activation energy for the outer-sphere model calculated from the Marcus model is nearly the same as that obtained by a direct calculation of the precursor- and transition-state energy. A simple model with one water ligand is shown to recover 60% of the reorganization energy. This finding is important because it indicates the possibility to carry out theoretical studies of electron-transfer reactions involving M 3+ and M 4+ actinide species that have eight or nine water ligands in the first coordination sphere.