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

摘要

The formation of ternary UO_2(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 ~(19)F NMR spectroscopy. The equilibrium constants have been determined from the emf data in a NaClO_4 medium at constant sodium concentration, [Na~+]=1.00 M at 25 deg C, except for the UO_2(ac)F_3~(2-) complex where ~(19)F NMR at -5 deg 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 UO_2(ac)F_3~(2-) complex was studied using ~(19)F 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 intermadiate UO_2F_3(H_2O_2)_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 UO_2(ox)_2(H_2O)~(2-) and UO_2(ac)_2(H_2O) have been studied using ~(13)C NMR techniques at -5 deg C. The rate law is #nu#=#kappa#[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 UO_2(H_2O)_5~(2+). The information from the binary oxalate system rules out the formation of UO_2(ox)_2(H_2O)~(2-) as an intermediate in the exchange reactions in the previously studied UO_2(ox)_2F~(3-), also in the 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.