The complexation reaction of macrocyclic ligand (4 /-nitrobenzo-15C5) with Y3+cation was studied in pure acetonitril,dimethylformamide, ethylacetate,methanol and also in acetonitrile- dimethylformamide (AN–DMF) and ethylacetate- methanol (EtOAc-MeOH) binary mixtures at different temperatures using conductometry method. The conductance data show that in all solvent systems, the stoichiometry of the complex formed between (4 /-NB-15C5) and Y3+cation is 1:1 (ML). The stability order of (4’-NB-15C5.Y)3+ complex in pure non-aqueous solvents at 25°C was found to be: EtOAc > AN ≈ DMF> MeOH and in the case of most compositions of binary mixed solutions at 25°C it was: AN-DMF > EtOAc-MeOH.The results show that the sequence of the stability of the complex in binary mixed solutions changes with temperature. A non-linear behaviour was observed for changes of log Kf of (4’-NB-15C5. Y)3+ complex versus the composition of the binary mixed solvents, which was explained in terms of solvent–solvent interactions and also the heteroselective solvation of the species involved in the complexation reaction. The values of thermodynamic parameters (ΔHºc and ΔS ºc ) for formation of the complex wereobtained from temperature dependent of the stability constant using the van‘t Hoffplots. The results show that in most cases, the complex is both enthalpy and entropy stabilized and the values and also the sign of thermodynamic parameters are influenced by the nature and composition of the mixed solvents.

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