The kinetics of interaction between glycylglycine and cis-[Ru(bipy) 2 (H 2 O) 2 ] 2+ have been studied spectrophotometrically as a function of [Ru(bipy) 2 (H 2 O) 2 2+ ], [diglycine] and temperature at a particular pH (4.8), where the substrate complex exists predominantly as the diaqua species and diglycine as the zwitter ion. The reaction has been found to proceed via two consecutive steps. The first step involves the ligand-assisted anation, while the second step involves chelation when the second aqua ligand is displaced. Rate constants have been evaluated and activation parameters calculated. The low enthalpy of activation and large negative values of entropy of activation indicate an associative mode of activation for both steps.

The kinetics of the interaction of thiosemicarbazide with cis-[Ru(bipy) 2 (H 2 O) 2 ] 2+ (bipy = a a¢-bipyridyl) have been studied spectrophotometrically as a function of [Ru(bipy) 2 (H 2 O) 2 2+ ], [bipyridyl] and temperature, at a particular pH (4.8), where the substrate complex exists predominantly as the diaqua species and thiosemicarbazide as the neutral ligand. The reaction proceeds via an outer sphere association complex formation, followed by two slow consecutive steps. The first is the conversion of the aforementioned complex into the inner sphere complex, and the second step involves the entrance of another thiosemicarbazide molecule in the coordination zone of Ru(II) whereby, in each step, an aqua ligand is replaced. The association equilibrium constant (K E) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. Activation parameters have been calculated for both steps using the Eyring equation (DH 1 # = 25.37±1.6 kJ mol )1 , DS 1 # = )215.48 ± 4.5 J K )1 mol )1 , DH 2 # = 24.24 ± 1.1 kJ mol )1 , DS 2 # = )207.14 ± 3.0 J K )1 mol )1 ). The low enthalpy of activation and large negative value of entropy of activation indicate an associative mode of activation for both aqua ligand substitution processes. From the temperature dependence of K E , the thermodynamic parameters calculated are: DH 0 = 10.75±0.54 kJ mol )1 and DS 0 = 84.67 ± 1.75 J K )1 mol )1 , which give a negative DG 0 value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex prior to the first step.

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The kinetics of interaction between L-asparagine and cis-[Ru(bipy) 2 (H 2 O) 2 ] 2+ have been studied spectrophotometrically as a function of [Ru(bipy) 2 (H 2 O) 2+ 2 ], [Lasparagine] and temperature at a particular pH 4.8 where the substrate complex exists predominantly as a diaqua species and L-asparagine as a zwitterion. The reaction has been found to proceed through two consecutive steps. The first step involves the ligand assisted anation while the second step involves chelation when the second aqua ligand is displaced. Rate constants have been evaluated and activation parameters are calculated. The low enthalpy of activation and large negative value of entropy of activation indicate an associative mode of activation for both the steps.

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