The complexation reaction between Mn2+,Co2+ ,Y3+ and ZrO2+ metal cations with the macrocyclic ligand, 4`-nitrobenz-15-crown-5 in pure acetonitrile, methanol and also acetonitrile-mathanol(AN-MeOH) binary mixtures have been studied at different temperatures using conductometry method. The values of formation constants of the complexes were obtained from fitting of molar conductivity curves using a computer program, GENPLOT [1,2]. The conductance data show that the stoichiometry of the complexes formed between 4`-NB15C5 with Mn+2,Co+2 ,Y +3 and ZrO+2 cations is 1:1[ML]. The values of the standard enthalpy changes (ΔH◦ c ) for complexation reactions were obtained from the slope of the v’ant Hoff plots and the changes in standard entropy ΔS◦ c were calculated from the relationship ΔG◦ C,298.15=ΔH◦ C-298.15ΔS◦ C[3]. In most cases, a non-linear relationship is observed between the stability constants of the complexes ( ) and the composition of the AN-MeOH binary solution. The order of stability of the metal–ion complexes in pure AN at 25 ◦C was found to be:(4`-NB15C5.Y)3+ >(4`NB15C5.Mn)2+ >(4`-NB15C5.Co)2+>(4`-NB15C5.ZrO)2+ and in the case of pure MeOH at the same temperature, it changes to:(4`-NB15C5.Co)2+>(4`-NB15C5.Mn)2+>(4`-NB15C5.ZrO)2+>(4`-NB15C5.Y)3+. The results also show that the stability sequence of the complexes in binary solution of AN-MeOH (mol MeOH =75) at 25 (4`-NB15C5.ZrO)2+>(4`-NB15C5.Co)2+>(4`- NB15C5.Y)3+>(4`NB15C5.Mn)2+ and in AN-MeOH binary solvent (mol MeOH=50), it is: (4`NB15C5.Mn)2+ >(4`-NB15C5.ZrO)2+>(4`- NB15C5.Y)3+>(4`-NB15C5.Co)2+ and in the case of AN-DMF(mol MeOH=25) at 25 it changes to: (4`-NB15C5.Co)2+>(4`-NB15C5.ZrO)2+≈ (4`- NB15C5.Y)3+>(4`NB15C5.Mn)2+. The experimental results show that depending on the nature and composition of the binary solvent systems, the selectivity of the ligands for the metal cations and thermodynamic of complexation processes may be changed[4,5].

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