KINETICS AND EQUILIBRIUM OF REACTION BETWEEN OCTAKIS (DIMETHYL-SULFOXIDE) URANIUM(IV) AND NITRITE ION IN DIMETHYL-SULFOXIDE

The kinetics and equilibrium of the reaction between octakis (dimethyl sulfoxide) uranium (IV), U(dmso)8(4+), and nitrite ion NO2- 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 NO2-. The first reaction is the formation of 1 : 2 adduct complex [U(dmso)8(NO2)2]2+, the rate of which was so fast that the rate constant k1 obs was not determined accurately even by the stopped-flow method. If the concentration of NO2- 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 NO2- under this condition. The second reaction represents the substitution of coordinated dmso with NO2- forming mixed ligand complex U(dmso)8-x(NO2-)x+2](2-x)+. The rate constant k2 obs for the second reaction was measured by the conventional method using a diode-array spectrophotometer and k2 obs increased linearly with the concentration of free NO2-. The third reaction coincides with the intra-molecular oxygen transfer from coordinated NO2- to U(IV) and this process is accompanied by the oxidation of U(IV) to U(VI). The oxygen transfer mechanism was confirmed by O-17 labeled experiments using O-17-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).