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The kinetics of the interaction between thiourea and the title complex has been studied spectrophotometrically in aqueous medium as a function of nucleophile concentration, temperature and pH at constant ionic strength. The reaction is a two-step process in which the first step is ligand dependent, but the second step is ligand independent and is assigned to ring closure. The rate and activation parameters, conductivity studies and IR data were used to deduce a plausible mechanism.

The kinetics of the aqua ligand substitution from hydroxopentaaquarhodium(III) ion, [Rh(H 2 O) 5 (OH)] 2? , by L-Arginine has been studied spectrophotometrically as a function of Arginine concentration, and temperature, at pH 4.3. The reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyring's equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation. The product of the reaction has been characterized by conductivity measurement and IR spectroscopic analysis.

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The kinetics of the interaction of glycine-Lleucine (Glyleu) with cis-[Pt(cis-dach)(OH 2 ) 2 ] 2? (dach = 1, 2-diaminocyclohexane) has been studied spectrophotometrically as a function of [cis-[Pt(cis-dach)(OH 2 ) 2 ] 2? ], [Glyleu] and temperature at pH 4.0, where the complex exists predominantly as the diaqua species and Glyleu as a zwitterion. The substitution reaction shows two consecutive steps: the first is the ligand-assisted anation and the second is the chelation step. The activation parameters for both the steps were evaluated using Eyring's equation. The low DH 1 = (51.9 ± 2.8 kJmol -1 ) and large negative value of DS 1 = (-152 ± 8 JK -1 mol -1 ) as well as DH 2 = (54.4 ± 1.7 kJmol -1 ) and DS 2 = (-162 ± 5 JK -1 mol -1 ) indicate an associative mode of activation for both the aqua ligand substitution processes.

The kinetics of interaction between cis-[RuCl(Me 2 SO) 3 (H 2 O) 2 ] + and some selected dipeptides such as glycyl glycine(gly gly), glycyl alanine(Gly-L-ala) and glycyl-L-leucine(gly leu) has been studied spectrophotometrically as a function of [RuCl(Me 2 SO) 3 (H 2 O) + 2 ], [dipeptide] and temperature at a particular pH(5.0), where the substrate complex exists predominantly as a diaqua species (in aqueous solution) and dipeptides as the zwitter ions. The reaction has been found to proceed via two distinct consecutive steps i.e., it shows a non-linear dependence on the concentration of dipeptide: first process is dependent and the second step is independent of ligand concentration respectively. The rate constants for the processes are: k 1 ∼ 10 −3 s −1 and k 2 ∼ 10 −5 s −1. The activation parameters were calculated from Eyring plots suggests an associative mechanism for the interaction process. From the temperature dependence of the outer sphere association equilibrium constants, the thermodynamic parameters were also calculated, which gives a negative G 0 value at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. Keywords. Ligand substitution; dipeptides; cis-[RuCl(Me 2 SO) 3 (H 2 O) 2 ] + ; kinetics.

The interaction of thiosemicarbazide with the title complex has been studied spectrophotometrically in aqueous medium as a function of [complex], [thiosemicarbazide], pH and temperature at constant ionic strength. At pH 7.4, the reaction shows two distinct paths; both of which are [thiosemicarbazide] dependent. A parallel reaction scheme fits well with the experimental findings. An associative interchange mechanism is proposed for both the paths; the activation parameters calculated from Eyring plots are DH 1 = = 14.2 ± 0.8 kJ mol -1 , DS 1 = = -241 ± 2 JK -1 mol -1 , DH 2 = = 30.8 ± 1.4 kJ mol -1 and DS 2 = = -236 ± 4 JK -1 mol -1 . From the temperature dependence of the outer sphere association complex equilibrium constants, the thermodynamic parameters calculated are DH 1°= 34.25 ± 1.9 kJ mol -1 , DS 1°= 146 ± 6 J K -1 mol -1 and DH 2°= 9.4 ± 1.1 kJ mol -1 , DS 2°= 71 ± 3 JK -1 mol -1 , which gives a negative DG°at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex.

The kinetics of the interaction between diethyldithiocarbamate (Et 2 DTC) and the title complex has been studied spectrophotometrically in aqueous medium as a function of nucleophile concentration, temperature and pH at constant ionic strength. The reaction is a two-step process in which the first step is liganddependent, but the second step is ligand-independent and is assigned to ring closure. The rate and activation parameters, conductivity studies and IR data were used to deduce a plausible mechanism.