The complexation reaction between Y3+ cation and macrocyclic ligand, 40-nitrobenzo- 18-crown-6 (40NB18C6), was studied in acetonitrile–methanol (AN–MeOH), acetonitrile–1, 2-dichloroethane (AN–DCE), acetonitrile–dimethylformamide (AN–DMF) and acetonitrile– ethylacetate (AN–EtOAc) binary mixed solvent solutions at different temperatures using the conductometric method. The conductance data show that in most cases, the stoichiometry of the complex formed between 40NB18C6 and Y3+ cation is 1:1 [M:L], but in the case of AN-DCE binary solution (mol% DCE = 50) at 15, 25 and 35 C, a 2:1 [M2:L] and also a 2:2 [M2:L2] complexes are formed in solution. The results show that the stoichiometry of the complex formed between 40NB18C6 and Y3+ cation changes with the composition of the mixed solvents and even with temperature. The stability constant of the 1:1 complex was determined using a computer program, GENPLOT. The stability order of (40NB18C6.Y)3+ complex in pure studied solvents at 25 C was found to be: EtOAc >AN > MeOH> DMF and in the case of the mixed solvent solutions with 25 mol percent of AN at 25 C was: AN-DCE > AN-EtOAc > AN-MeOH  AN-DMF. The values of stability constant (logKf) of (40NB18C6.Y)3+ complex which were obtained from conductometric data, show that the stability of the complex is not only affected by the nature and composition of the solvent system, but it is also influenced by the temperature. In all cases, a non-linear behavior is observed for changes of logKf of the (40NB18C6.Y)3+ complex versus the composition of the binary mixed solvents. The values of standard thermodynamic quantities (DHc and DSc) for the complexation process which were obtained from temperature dependence of the stability constant of (40NB18C6.Y)3+ complex, show that depending on the solvent system, in most cases, the complex is enthalpy and also entropy stabilized, but in some cases, it is stabilized or destabilized from these view points.

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