Equilibria and dynamics in binary and ternary uranyl oxalate and acetate/fluoride complexes†

Abstract

The formation of ternary UO₂(ac)_pF_q^2–p–q (p = 1 or 2 and q = 1–3) complexes, and their equilibrium constants were investigated by potentiometric titrations and ¹⁹F NMR spectroscopy. The equilibrium constants have been determined from the emf data in a NaClO₄ medium at constant sodium concentration, [Na⁺] = 1.00 M at 25 °C, except for the UO₂(ac)F₃^2– complex where ¹⁹F NMR at –5 °C was used. The magnitude of the equilibrium constant for the stepwise addition of fluoride indicates that prior co-ordination of acetate has only a small effect on the subsequent bonding of fluoride. The acetate exchange in the ternary UO₂(ac)F₃^2– complex was studied using ¹⁹F NMR. Through magnetisation transfer experiments, it was possible to confirm the provisional mechanism from a previous study and also the consistency of the rate constants for the five different exchange pathways required to describe the fluoride exchange. The exchange takes place via the intermediate UO₂F₃(H₂O)₂^–, indicating that the acetate exchange follows an interchange mechanism with solvent participation in the transition state. The rates and mechanisms of the ligand exchange reactions in UO₂(ox)₂(H₂O)^2– and UO₂(ac)₂(H₂O) have been studied using ¹³C NMR techniques at –5 °C. The rate law is v = k[complex][ligand], and the second order rate constant and the activation parameters for both systems have been determined. The reactions most likely take place through an Eigen–Wilkins type of mechanism, where the first step is a pre-equilibrium of an outer-sphere complex followed by a rate determining exchange of water. The rate constants for the water exchange reactions are very similar to that in UO₂(H₂O)₅²⁺. The information from the binary oxalate system rules out the formation of UO₂(ox)₂(H₂O)^2– as an intermediate in the exchange reactions in the previously studied UO₂(ox)₂F^3–, also in this case confirming a previously suggested exchange mechanism. The H⁺/D⁺ isotope effects and a linear free energy relationship suggest that the main catalytic effect of H⁺ on ligand exchange rates is due to the formation of a protonated precursor. Hence, the catalytic effect depends on the basicity of the ligand and the site for the proton attack.