Kinetic parameters for the reactions of U(III) with Ru(NH3)63+, Ru(NH3)5OH23+, and Ru(en)33+ in trifluoromethanesulfonate media

The rates of the reactions between U(III) and aquopentaammineruthenium(III), hexaammineruthenium(III), and tris(ethylenediamine)ruthenium(III) are reported. In all three reactions the observed rates show a first-order dependence on each reactant. Over the acid range investigated ([H]+ = 0.02-1.0 M), except for Ru(en)33+, no acid dependence is observed and over the temperature range investigated (5-400C), no temperature dependence is observed. The reaction parameters are as follow: In 1 M HCF3SO3, k = (1.11 +- 0.33) x 104, ΔH2+ = -0.6 +- 1.4, and ΔS2+ = -38 +- 5 for U3+ + Ru(NH3)5OH23+; k = (1.10 +- 0.46) x 104, ΔH2+ = 1.0 +- 0.7, and ΔS2+ = -38 +- 2 for U3+ + Ru(NH3)63+; in 0.1 M HCF3SO3, k = 1.62 +- 0.27 x 105, ΔH2+ = -1.3 +- 1.3, and ΔS2+ = -39 +- 4 for U3+ + Ru(en)33+. The small enthalpies of activation are rationalized in terms of the Coulombic energy and solvent reorganization energy required to form the precursor complex. Ionic strength parameters are of the magnitudes expected for reactions between two 3+ species. Rate constants derived from perchlorate media are similar to those from trifluoromethanesulfonate media, indicating that the two anions behave similarly even for reactions between highly charged species.The experimental results are used to calculate self-exchange rates for the other reactants. The calculated electron self-exchange rates are 5.9 x 10-5 for U/sup 4+/3+/, 3.6 x 104, for Ru(en)3/sup3+/2+/, and 2.9 x 103 for Ru(NH3)5OH2/sup 3+/2+/