Spectrophotometric and potentiometric study on platinum(II) chelates of mercapto carboxylic acids

The coordination of Pt(II) with different mercapto and dimercapto compounds has been studied using both potentiometric titration at 25 degreesC in KCl 0.1 M and UV-Vis spectroscopy. The slow complex formation kinetics has been followed spectrophotometrically for several days on samples with different ligand molar fractions ranging from 0.1 to 1, keeping the ligand and metal concentration below 8 x 10(-4) M. For the potentiometric titration, the ligand concentrations were kept constant at 10(-3) M, and the 0.25, 0.5, 0.75 and 1.00 metal/ligand ratios were titrated in the pH range 2.5-11 for each ligand. The solutions were kept in a thermostatted bath at 25 degreesC for 2 days before titration. The stabilization of the absorbance values under these conditions after 2 days, and the back titration with hydrochloric acid 0.1 M, led us to conclude that the proper equilibrium complex formation is reached under these conditions. Using KCl 0.1 M as an ionic strength medium, the reported formation constants are referred to the complexation reaction starting from [PtCl4](2-). Some models are proposed on the basis of the estimated stoichiometries. (C) 2000 Elsevier Science B.V. All rights reserved.