KINETICS OF LIGAND-EXCHANGE REACTIONS FOR URANYL(2-SOLUTION() FLUORIDE COMPLEXES IN AQUEOUS)

Rates and mechanisms of fluoride exchange reactions between various ur anyl fluoro complexes {UO2(H2O)(5-n)F-n(2-n)}, and HF/F- have been stu died in aqueous solution using F-19 and O-17 NMR line broadening techn ique. A group of 15 different exchange pathways has been identified, a nd their rate laws and rate constants have been determined. All reacti ons 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 int ermediate/transition state, e.g., UO2F+ + UO2F-2 reversible arrow UO2 FF + UO2F+ (k(1,2)), and fluoride exchange between a uranyl complex a nd F-/HF, e.g., UO2F+ + HF reversible arrow UO2*F+ + HF (k(1,HF)) The exchange between UO22+ and UO2F+ takes place mainly according to UO22 + + HF reversible arrow UO2F+ + H+ (forward, k'(0,HF); reverse, k(1,HF )) Most of these reactions have rate constants, k(m,n) approximate to 5 x 10(4) M(-1) s(-1), at -5 degrees C. The exchange reactions seem to follow the Eigen-Wilkins mechanism, where the rate determining step i s a ligand promoted dissociation of coordinated water. The exchanges i nvolving UO2Fn2-n, n = 4 and 5, are much faster than the others, indic ating mechanistic differences. The exchange rate was approximately 3 t imes faster for reactions involving DF than for HF. The activation par ameters have been determined for two reaction pathways.