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

Viljoen, Karen (Stellenbosch : University of Stellenbosch, 2003-12)

Thesis

ENGLISH ABSTRACT: Ruthenium, as one of the platinum group metals, was investigated to determine the aquation rate constant of [RuCl6]3- and the anation rate constant of [RuCl5(H2O)]2-. This two reactions represent the equilibrium reaction [RuCl6]3- + H2O ⇄ [RuCl5(H2O)]2- + 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) x10-3 s-1 at 25°C. The activation energy, Ea, is 90.1 (±1.2) kJ.mol-1 and the enthalpy 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 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) x10-3 M-1s-1. The activation energy is 88.0 (±1.4) kJ.mol-1, with the enthalpy and entropy of 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 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-1 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]3- 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 Å. 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 Å – 2.375 Å). The diethylenetriamine crystal structures compared well. The conclusion was that the crystals prepared were diethylenetriamine hexachlororuthenate(III).

AFRIKAANSE OPSOMMING: Ruthenium(III), een van die platinum groep metaal-ione, is in hierdie studie ondersoek om die akwasie tempo konstante van [RuCl6]3- en die anasie tempo konstante van [RuCl5(H2O)]2- te bepaal. Dié twee reaksies verteenwoordig die ewewigsreaksie [RuCl6]3- + H2O ⇄ [RuCl5(H2O)]2- + Cl-. Die verloop van die reaksies is met behulp van UV/Sigbare spektroskopie by verskillende temperature gevolg. Die akwasie en anasie tempo konstantes is bepaal met goeie presisie en die termodinamiese konstantes van die reaksies is bereken. Die pseudo-eerste orde akwasie tempo konstante, k65, is bepaal deur middel van regressie, as 52.1 (±3.7) x10-3 s-1 by 25°C. Die aktiverings energie, Ea, is bereken as 90.1 (±1.2) kJ.mol-1 en die entalpie en entropie van aktivering is onderskeidelik 87.7 (±1.2) kJ.mol-1 en 24.7 (±4.3) J.K-1.mol-1. Daar is gevind dat die akwasie reaksie konstante afhanklik was van die soutsuur konsentrasie: dit neem af soos die soutsuur konsentrasie toeneem. Met behulp van die regressie lyn is die anasie tempo konstante bepaal by 25°C as 1.62 (±0.11) x10-3 M-1s-1. Die aktiveringsenergie is bepaal as 88.0 (±1.4) kJ.mol-1 en die entalpie en entropie van aktivering, onderskeidelik as 85.6 (±1.4) kJ.mol-1 en –11.2 (±4.7) J.K-1.mol-1. Die invloed van die soutsuur konsentrasie op die anasie tempo konstante is nie bepaal nie. Die ewewigskonstante vir die reaksie wat ondersoek is, is bereken met die tempo konstantes vir die akwasie en anasie reaksies. Die ewewigskonstante, K6, is bereken as 0.0311 M-1 by 25°C. Toe die ewewigskonstante vergelyk is met die literatuur waardes, is gevind dat die ewewigskonstante afhanklik is van die soutsuur konsentrasie. Saam met die waardes wat in die literatuur gevind is, is die ewewigskonstante gebruik om twee distribusie diagramme te bereken. Die distribusie diagramme vir die Ru(III) spesies toon onderskeidelik 79.9% en 72.3% [RuCl6]3- in 12M HCl. Die twee distribusie diagramme is baie eenders en dit is nie moontlik om ‘n finale distribusie diagram op te trek totdat die uitstaande tempo konstantes tussen die akwachloro Ru(III) spesies bepaal word nie. Die tempo konstantes en termodinamiese waardes wat bepaal is vir die Ru(III) reaksie is vergelyk met gelyksoortige waardes in die literatuur van Rh(III) en Ir(III) omdat daar ooreenkomste tussen die platinum groep metale opgemerk is. Daar is bevind dat die akwasie én anasie reaksies die volgende patroon volg: Ru(III) > Rh(III) > Ir(III). Die verskille word in sekere gevalle benut in die raffinering van hierdie metale. Kristalle van dietileentriamien heksachlororuthenaat(III) is berei en gekarakteriseer met behulp van CHN analise en x-straal kristallografie. Die gemiddelde Cl-Ru bindingsafstand vir die kristal was 2.371 Å. Die kristalstruktuur is vergelyk met dié van heksaäkwaäluminium hexachlororuthenaat(III) tetrahidraat en diëtileentriamien heksachlororhodaat(III). Die chloried-metaal bindingsafstand vir die kristalle was soortgelyk (2.350 Å – 2.375 Å). Die diëtileentriamien kristalstrukture stem goed ooreen. Die gevolgtrekking was dat die kristalle wat voorberei is wel diëtileentriamien heksachlororuthenaat(III) was.

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