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

Ruthenium, as one of the platinum group metals, was investigated to determine the aquation rate constant of [RuCl6] and the anation rate constant of [RuCl5(H2O)]. This two reactions represent the equilibrium reaction [RuCl6] + H2O ⇄ [RuCl5(H2O)] + Cl. The reactions were followed, using stopped-flow injection and UV/Visible spectroscopy, at different temperatures. The aquation and anation rate constants were determined with good precision and thermodynamic values for the reactions were calculated. The pseudo first order aquation rate constant, k65, was determined by calculation from the regression line as k65 = 52.1 (±3.7) x10 s at 25°C. The activation energy, Ea, is 90.1 (±1.2) kJ.mol and the enthalpy and entropy of activation is 87.7 (±1.2) kJ.mol and 24.7 (±4.3) J.K.mol, respectively. The aquation rate constant was found 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 rate constant, k56, was calculated as 1.62 (±0.11) x10 Ms. The activation energy is 88.0 (±1.4) kJ.mol, with the enthalpy and entropy of activation 85.6 (±1.4) kJ.mol and –11.2 (±4.7) J.K.mol, respectively. The influence of the hydrochloric acid concentration of the solution on the anation rate constant was not investigated. The equilibrium constant for the reaction studied was calculated from the rate constants for the aquation and anation reactions. The equilibrium constant, K6, was calculated as 0.0311 M at 25°C. The equilibrium constant, when compared to literature, was found to be dependent on the hydrochloric acid concentration. It was then used, in conjunction with data from the literature, to construct two distribution diagrams. Distribution diagrams for the Ru(III) aquachloro species show between 79.9% to 72.3% [RuCl6] present in 12M HCl. The two distribution diagrams were very similar and it is not possible to resolve the issue of a final distribution diagram for the aqua-chloro Ru(III) system without further investigation into the all the other rate constants of the Ru(III) aqua-chloro species. The rate constants and thermodynamic values for the Ru(III) reaction were compared to corresponding data (from literature) for Rh(III) and Ir(III) because several comparisons between these platinum group metals have been noted. It was found that for both the aquation and anation rate constants, the following trend was observed: Ru(III) > Rh(III) > Ir(III). These differences are in certain cases exploited in the refining of these platinum group metals. Crystals of diethylenetriamine hexachlororuthenate(III) was prepared and characterised by x-ray crystallography and CHN analysis. The average Cl-Ru bond length for the crystal was 2.371 A. The crystal structure was compared to hexaaquaaluminium hexachlororuthenate(III) tetrahydrate and diethylenetriamine hexachlororhodate(III). The metal-chloride bond lengths of all the crystals were found to be similar (2.350 A – 2.375 A). The diethylenetriamine crystal structures compared well. The conclusion was that the crystals prepared were diethylenetriamine hexachlororuthenate(III).