The kinetics of anation of the hydroxopentaaquarhodium(III) ion by pyridine-2-aldoxime have been studied spectrophotometrically as a function of pH, substrate concentration, entering ligand, temperature and solvent polarity. The reaction rate increases with an increase in pH, temperature and ligand concentration, and reaches a limiting value at high ligand concentration. The activation parameters (AH * = 87.5 kJmol-1 and AS s = -52.3 J K-1 tool-1) have been calculated from an Eyring plot and are compared with aqua exchanges in other systems. All the experimental facts lead to an associative interchange (Ia) mechanism with the prior formation of an outer-sphere association complex. Solvent polarity effects lead to the same conclusion.

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The kinetics of the interaction of adenosine with [Pt(en)(H20)2] 2 ' has been studied spectrophotometrically as a function of [Pt(en)(H20)2 + ], [Adenosine] and temperature at a particular pH (4.0) where the substrate complex exists predominantly as the diaqua species and the ligand adenosine as a neutral molecule. The interaction reaction shows distinct two step consecutive process; the first step is the ligand assisted anation and the second step is the chelation step. The activation parameters for both the steps are calculated (AHf = 63.7 ± 1.6kj mol ASj" = -58± 5JK 1 mol 1 and AH2 = 41.0 ± 0.5 kj mol \ AS2 = 197 ± 1.5 JK 1 mol 1 ).