The complexation reactions between 4-(2-pyridy1azo)-resorcinol and Cd ,H g and Pb io ns have been investigated in acetonitrile-dimethyEomamide mixtures at various temperatures using a spectrophotometric technique. The stoichiometry and
the formation constants of the complexes formed in solution were calculated by
computer refinement of absorbance-mole ratio data using nonlinear least-square
program. The stoichiometries of resulting complexes are 1 : 1,l: 1 -2: 1 and 1 :2 (metalto - ligand ratio) for Pb H, g a nd Cd r espectively. Linear relationship is observed between equilibrium constants and the mole fraction of aceionitrile in the solvent mixtures. The ?H? and AS? of complex formation reactions were calculated from the temperature dependence of the stability constants. The effects of solvent composition on stability and thermodynamic parameters were discussed