KINETICS AND MECHANISM OF LIGAND SUBSTITUTION OF AQUOPROPYLENE-DIAMINETETRAACETATO RUTHENATE(III) IN WATER

The reaction of pentadentate PDTA complex of ruthenium(III) with various sulphur containing ligands such as thiourea and 2-mercaptopyrimidine were studied as a function of pH (1.5-9.0), temperature (30-45-degrees-C) and substituting ligand concentration at 0.2 M ionic strength (NaClO4) in water. The reaction of Ru(PDTA)(H2O)- with 2-mercaptopyrimidine proceeds with the formation of a red product (lambda-max = 511 nm) which undergoes pH-dependent conversion into a violet product (lambda-max = 541 nm). The activation parameters at pH 5 lie in the range 21.9 < DELTA-H not-equal < 28.8 KJ mol-1 and -120.4 < DELTA-S not-equal < -96.7 J mol-1 deg-1 and support the operation of an associatively activated ligand substitution. The experiments are discussed with reference to data for the corresponding ethylenediaminetetraacetate complex and the labilisation effect of such chelated ligands.