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The stability constants (K f ) for the complexation reactions of Cr 3? , Mn 2? and Zn 2? metal cations with macrocyclic ligand, 15-crown-5 (15C5), in acetonitrile (AN), ethanol (EtOH) and also in their binary solutions (AN-EtOH) were determined at different temperatures, using conductometric method. 15C5 forms 1:1 complexes with Cr 3? , Mn 2? and Zn 2? cations in solutions. A nonlinear behaviour was observed for changes of logK f of the metal ion complexes versus the composition of the mixed solvent. The order of stability of the metal-ion complexes in pure AN and in a binary solution of AN-EtOH (mol% AN = 52) at 25°C was found to be: (15C5Zn) 2? [ (15C5ÁMn) 2? [ (15C5ÁCr) 3? , but in the case of pure EtOH at the same temperature, it changes to: (15C5ÁZn) 2? [ (15C5ÁCr) 3? [ (15C5ÁMn) 2? . The results also show that the stability sequence of the complexes in the other binary solutions of AN-EtOH (mol% AN = 26 and mol% AN = 76) varies in order: (15C5ÁCr) 3? * (15C5ÁZn) 2? [ (15C5ÁMn) 2? . The values of the standard thermodynamic quantities (DH C°, DS C°) for formation of (15C15-Cr 3? ), (15C5-Mn 2? ) and (15C5-Zn 2? ) complexes were obtained from the temperature dependence of the stability constants and the results show that the thermodynamics of complexation reactions is affected by nature and composition of the solvent systems and in most solution systems, the complexes are enthalpy stabilized but entropy destabilized.

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