A different behavior was observed for complexation of Y+3 cation with 4′NB18C6 in AN-DCE binary system. As is clear from Fig. 2, addition of the ligand to Y+3 cation in AN-DCE (mol%DCE = 50) binary solution at 15, 25 and 35 °C, causes the molar conductivity to initially decrease until the mole ratio reaches to about and then increases. The behavior observed in this solvent system, probably shows that a 2:1 [M2:L] complex is formed between 4′NB18C6 and Y+3 cation in the solution which is less mobile than the free solvated Y+3 cation. It seems that further addition of the ligand to [M2:L] complex solution, causes the formation of a [M2:L2] complex in solution which is less solvated than [M2:L] complex and, therefore, the molar conductivity increases. Since the cavity size of 4′NB18C6 is not big enough to fit two Y+3 cations, but it may suggest that the second Y+3 cation probably interacts with the ligand via oxygen donor of the –NO2 group and also with the π electron system of benzo group and, therefore, it forms a 2:1 [M2:L] complex with the metal cation in the solution and further addition of 4′NB18C6 results in the formation a [M2:L2] complex with a club sandwich structure. It is interesting to note that at 45 °C, a relatively stable 1:1 [M:L] complex is formed in this solvent system. The results obtained in this study show that the stoichiometry of the complex formed between 4′NB18C6 and Y3+ cation changes with the composition of the binary mixed solvent solution and also with temperature.