VARIABLE-PRESSURE H-1-NMR STUDIES OF HEXAMETHYLPHOSPHORAMIDE AND TRIMETHYL PHOSPHATE EXCHANGE ON DIOXOURANIUM(VI)

By extending the variable pressure H-1 NMR studies to the actinides, v olumes of activation and new variable-temperature activation parameter s are obtained for solvent exchange in CD2Cl2 diluent for [UO2(HMPA)4] 2+: k2 (273 K)=40.4 +/- 3 kg mol-1 s-1, DELTAH2 double dagger = 47.1 /- 3 kJ mol-1, DELTAS2 double dagger = -41.0 +/- 9 J K-1 mol-1, DELTAV 2 double dagger -11.3 +/- 1.4 cm3 mol-1, and for [UO2(TMPA)5]2+: k1 (2 60 K) = 55.7 +/- 3 s-1, DELTAH1 double dagger = 44.1 +/- 2 kJ mol-1, D ELTAS1 double dagger = -40.7 +/- 6 J K-1 mol-1, DELTAV1 double dagger = +2.1 +/- 1.5 cm3 mol-1. These new rate parameters which complete our earlier studies, indicate the operation of A and D mechanisms for exc hange of HMPA and TMPA, respectively, on the dioxouranium(VI) ion.