Kinetics and Equilibrium of Reaction between Octakis (dimethyl sulfoxide) uranium(IV) and Nitrite Ion in Dimethyl Sulfoxide

Abstract

The kinetics and equilibrium of the reaction between octakis (dimethyl sulfoxide) uranium (IV), U(dmso)8 4+, and nitrite ion N02 − were studied in dmso solutions by using stopped-flow, diode-array and UV-visible spectrophotometers. Three different reactions were observed under the condition of an excess NaNO2, and U(IV) was finally oxidized to U(VI) by N02 −. The first reaction is the formation of 1: 2 adduct complex [U (dmso8)(NO2)2 2+), the rate of which was so fast that the rate constant kl obs was not determined accurately even by the stopped-flow method. If the concentration of N02 − was as low as that of U(IV), 1:1 adduct complex was formed. However, 1:1 complex was inert for further redox process, i.e. U(IV) was not oxidized by N02 − under this condition. The second reaction represents the substitution of coordinated dmso with N02 − forming mixed ligand complex [U(dmso) 8-x(N02 −)x+2(2-x +) The rate constant k 20bs for the second reaction was measured by the conventional method using a diode-array spectrophotometer and k 20bs increased linearly with the concentration of free N02 −. The third reaction coincides with the intra-molecular oxygen transfer from coordinated N02 − to U(IV) and this process is accompanied by the oxidation of U(IV) to U(VI). The oxygen transfer mechanism was confirmed by 17O labeled experiments using 17O-NMR. It was found from acid dependence experiments that only nitrite ion oxidized U(IV) and that nitrous acid was inert as an oxidant for U(IV).