The rates and mechanisms of water exchange of actinide aqua ions: A variable temperature O-17 NMR study of U(H2O)(10)(4+), UF(H2O)(9)(3+), and Th(H2O)(10)(4+)

The rate constants and the activation parameters for the exchange between w ater solvent and [U(H2O)(10)](4+) and [UF(H2O)(9)](3+), and a lower limit f or the rate constant at room temperature for [Th(H2O)(10)](4+), were determ ined by O-17 NMR spectroscopy in the temperature range 255-305 K. The exper iments were made at different constant hydrogen ion concentrations, which v aried between 0.16 and 0.8 mol kg(-1). The Th(IV) system was investigated u sing Tb3+ as a shift reagent. The following kinetic parameters at 25 degree s C were obtained: k(ex) = (5.4 +/- 0.6) 10(6) s (-1), Delta H-not equal:= 34 +/- 3 kJ mol(-1), Delta S-not equal = -16 +/- 10 J mol(-1) K-1 for U4+(a q), k(ex) = (5.5 +/- 0.7) 10(6) s (-1), Delta H-not equal = 36 +/- 4 kJ mol (-1), Delta S-not equal = 3 +/- 15 J mol(-1) K-1 for UF3+(aq), and k(ex) > 5 10(7) s (-1) for Th4+(aq), where the uncertainty is given at the 2 sigma- level. This is the first experimental information on the kinetic parameters for the exchange of water for any M4+ ion. There is no information on the rates and mechanisms of ligand substitutions involving other mono-dentate l igands, hence the mechanistic interpretation of the data is by necessity pr ovisional. The kinetic data and the known ground-state geometry with a coor dination number of 10 +/- 1 for the Th(IV) and U(IV) complexes suggest a di ssociatively activated interchange mechanism. There is no noticeable effect of coordination of one fluoride or one hydroxide to U(IV) on the water exc hange rate. This is unusual, for other metal ions there is a strong labiliz ing of coordinated water when a second ligand is bonded, e.g., in complexes of aluminum and some d-transition elements. In previous studies of the rat es and mechanisms of ligand exchange in uranium(VI) systems we found a stro ng decrease in the lability of coordinated water in some fluoride containin g complexes.