Kinetics of Ligand Exchange Reactions for Uranyl(2+) Fluoride Complexes in Aqueous Solution

Abstract

Rates and mechanisms of fluoride exchange reactions between various uranyl fluoro complexes {UO2(H2O)5 - n F n 2- n }, and HF/F- have been studied in aqueous solution using 19F and 17O NMR line broadening technique. A group of 15 different exchange pathways has been identified, and their rate laws and rate constants have been determined. All reactions are first order with regard to the uranyl complex and second order overall. Two pathways dominate: fluoride exchange between two uranyl complexes, presumably through the formation of a fluoride bridging intermediate/transition state, e.g., UO2F+ + UO2*F2 ⇌ UO2F*F + UO2F+ (k 1,2), and fluoride exchange between a uranyl complex and F-/HF, e.g., UO2F+ + H*F ⇌ UO2*F+ + HF (k 1,HF). The exchange between UO2 2+ and UO2F+ takes place mainly according to UO2 2+ + HF ⇌ UO2F+ + H+ (forward, k‘0,HF; reverse, k 1,HF). Most of these reactions have rate constants, k m,n ≈ 5 × 104 M-1 s-1, at −5 °C. The exchange reactions seem to follow the Eigen−Wilkins mechanism, where the rate determining step is a ligand promoted dissociation of coordinated water. The exchanges involving UO2F n 2- n , n = 4 and 5, are much faster than the others, indicating mechanistic differences. The exchange rate was approximately 3 times faster for reactions involving DF than for HF. The activation parameters have been determined for two reaction pathways.

Solution coordination chemistry of uranium in the binary UO22+-SO42-and the ternary UO22+-SO42--OH-system

“…if they are of the Eigen-Wilkins type, or not. Nevertheless some observations based on the rate constants may be made: The rate constant k 1 is a factor of ten smaller than that observed for the exchange of fluoride between UO 2 F + and HF, which seems to follow an Eigen-Wilkins mechanism [22,24]. The outer-sphere equilibrium constant K os estimated using the Fuoss equation [23], is expected to be several orders of magnitude larger for reaction (1) than the exchange between UO 2 F + and HF in the fluoride system because of the different charges of the reactants.…”

“…The equilibrium constants were taken from the literature [3]. The rate constants summarized in Table 3 differ from those measured for the binary uranyl-fluoride system [22]. Szabó The experimental data do not allow an assignment of the intimate mechanisms, e.g.…”

“…Ref. [22] for notation, that is much smaller in the sulfate system than in the fluoride system. A ligand dependent value of k aq is reasonable if reaction (1) consists of several steps, as when a chelate is formed, as indicated by the EXAFS data.…”

Molecular simulation of the diffusion of uranyl carbonate species in aqueous solution

“…For the uranyl ion, Grenthe and co-workers (Szabó et al, 1996;Farkas et al, 2000) obtained water exchange rate constants of 1.4 Â 10 6 s À1 and subsequently 1.30 ± 0.05 Â 10 6 s À1 from 17 O NMR experiments. Consequently, the experimental values suggest that the activation free energy for water exchange is underestimated by the potential model, which predicts an exchange rate of 8.1 Â 10 8 s À1 .…”

Rates of Ligand Exchange around the Bis‐Oxalato Complex [NpO2(C2O4)2]3− Measured by Using Multinuclear NMR Spectroscopy under Neutral to Semi‐Alkaline Conditions

“…This ligand‐exchange reaction is important both technologically and geochemically because oxalate is the conjugate base of one of the simplest acids found in soils, in which it is a product of fungal metabolism, and oxalate is also used in the PUREX process for purifying uranium and plutonium . This research complements the existing study on homoleptic water‐exchange reactions for transition metals and studies of the carbonato complexes of other transuranic elements …”