Study of Complex Formation Between 18-Crown-6 with Cu2+, Zn2+, Cd2+ and Pb2+ Cations in Some Binary Mixed Non-aqueous Solvents Using the Conductometric Method

The complexation reactions between Cu2+, Zn2+, Cd2+ and Pb2+ metal cations with 18-crown-6 (18C6) were studied in methanol (MeOH) - dimethylformamide (DMF) and ethylacetate (EtOAc) - dimethylformamide (DMF) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complexes in all cases is 1:1 (ML). The values of thermodynamic parameters ( Hc 0, Sc 0) for 18C6-Cu2+, 18C6-Zn2+, 18C6-Cd2+ and 18C6-Pb2+ complexes in MeOH - DMF and EtOAc - DMF binary systems were obtained from temperature dependence of stability constants. The results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents and in most cases, the complexes are enthalpy stabilized but entropy destabilized. The stability constants Kf of all complexes increase with increasing the concentration of solvent with lower donicity (based on Gutmann donicity scale) in the binary mixed solvents. In most cases, a non-linear behaviour was observed for variation of logKf of the complexes with the composition of the mixed solvents. The selectivity order of 18C6 for these metal cations in all solvent systems used in this study is: Pb2+>Cd2+>Cu2+>Zn2+.