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

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Abstract

Rates and mechanisms of fluoride exchange reactions between various uranyl fluoro complexes {UO2(H2O)5-nFn2-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+ (k1,2), and fluoride exchange between a uranyl complex and F-/HF, e.g., UO2F+ + H*F ⇌ UO2*F+ + HF (k1,HF). The exchange between UO22+ and UO2F+ takes place mainly according to UO22+ + HF ⇌ UO2F+ + H+ (forward, k‘0,HF; reverse, k1,HF). Most of these reactions have rate constants, km,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 UO2Fn2-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.

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