19F Nuclear Magnetic Resonance Study on the Kinetics of the Fluoride Exchange in Uranium(VI) Fluoro Complex

Kinetics of the fluoride exchange in uranium(VI) (uranyl) fluoro complex was studied by using 19F NMR. The 19F NMR signal of coordinated fluoride at the equatorial positions of uranyl ion was first observed at about 80 ppm down field from that of bulk F−. From area measurements of the coordinated and free 19F NMR signals, penta-fluoro complex UO2F53− was confirmed to exist in aqueous solutions containing high concentration of fluoride ion. The stability constant K5 = [UO2F53−]/[UO2F42−][F−] was determined to be 1.31 mol−1 kg. The rate of F− exchange between coordinated and bulk F− was measured by the 19F NMR line-broadening method and the observed first-order rate constant kobsd was expressed as Kobsd = (k4 + k5K5[F−])[F−]/(1 + k5[F−]) where k4 and k5 are the exchange rate constants of F− between bulk and coordinated F− in UO2F42− and UO2F53−, respectively. The associative or associative interchange mechanism was proposed from the experimental results.