Ruthenium(III) aqua-chloro complex chemistry : the interconversion of the hexachlororuthenate(III) and aquapentachlororuthenate(III) species

[摘要] ENGLISH ABSTRACT: Ruthenium, as one of the platinum group metals, was investigated todetermine the aquation rate constant of [RuCl6]3- and the anation rateconstant of [RuCl5(H2O)]2-. This two reactions represent theequilibrium reaction [RuCl6]3- + H2O ⇄ [RuCl5(H2O)]2- + Cl-. Thereactions were followed, using stopped-flow injection and UV/Visiblespectroscopy, at different temperatures. The aquation and anationrate constants were determined with good precision andthermodynamic values for the reactions were calculated.The pseudo first order aquation rate constant, k65, was determined bycalculation from the regression line as k65 = 52.1 (±3.7) x10-3 s-1 at25°C. The activation energy, Ea, is 90.1 (±1.2) kJ.mol-1 and theenthalpy and entropy of activation is 87.7 (±1.2) kJ.mol-1 and 24.7(±4.3) J.K-1.mol-1, respectively. The aquation rate constant wasfound to be dependent on the hydrochloric acid concentration,decreasing with increasing hydrochloric acid concentration.From the regression line at 25°C the second order anation rateconstant, k56, was calculated as 1.62 (±0.11) x10-3 M-1s-1. Theactivation energy is 88.0 (±1.4) kJ.mol-1, with the enthalpy and entropyof activation 85.6 (±1.4) kJ.mol-1 and –11.2 (±4.7) J.K-1.mol-1,respectively. The influence of the hydrochloric acid concentration ofthe solution on the anation rate constant was not investigated.The equilibrium constant for the reaction studied was calculated fromthe rate constants for the aquation and anation reactions. Theequilibrium constant, K6, was calculated as 0.0311 M-1 at 25°C. Theequilibrium constant, when compared to literature, was found to bedependent on the hydrochloric acid concentration. It was then used, inconjunction with data from the literature, to construct two distributiondiagrams. Distribution diagrams for the Ru(III) aquachloro species show between 79.9% to 72.3% [RuCl6]3- present in 12M HCl. The twodistribution diagrams were very similar and it is not possible to resolvethe issue of a final distribution diagram for the aqua-chloro Ru(III)system without further investigation into the all the other rate constantsof the Ru(III) aqua-chloro species.The rate constants and thermodynamic values for the Ru(III) reactionwere compared to corresponding data (from literature) for Rh(III) andIr(III) because several comparisons between these platinum groupmetals have been noted. It was found that for both the aquation andanation rate constants, the following trend was observed: Ru(III) >Rh(III) > Ir(III). These differences are in certain cases exploited in therefining of these platinum group metals.Crystals of diethylenetriamine hexachlororuthenate(III) was preparedand characterised by x-ray crystallography and CHN analysis. Theaverage Cl-Ru bond length for the crystal was 2.371 Å. The crystalstructure was compared to hexaaquaaluminiumhexachlororuthenate(III) tetrahydrate and diethylenetriaminehexachlororhodate(III). The metal-chloride bond lengths of all thecrystals were found to be similar (2.350 Å – 2.375 Å). Thediethylenetriamine crystal structures compared well. The conclusionwas that the crystals prepared were diethylenetriaminehexachlororuthenate(III).

[发布机构] Stellenbosch University