Conductometric Studies of Solvent Influence upon Complex Formation Between 4'-Nitrobenzo-15-crown-5 and Ni~(2+),Co~(2+)and Cu~(2+)in Acetonitrile-Methanol Binary Mixtures

摘要

The complex formation between Ni~(2+),Co~(2+)and Cu~(2+)metal cations with 4'-nitrobenzo-15-crown-5 was studied in aceto-nitrile(AN)-methanol(MeOH)binary systems at different temperatures using conductometric method.In all cases,4'-nitrobenzo-15-crown-5 forms 1:1 complexes with these metal ions.The stability of the complexes is sensitive to the solvent composition and non-linear behaviour was observed for variation of log k_f of the complexes versus the composition of the binary mixed solvents.The results show that the selectivity order of 4'-mitrobenzo-15-crown-5 for the metal ions in pure AN is:Co~(2+)>Cu~(2+)>Ni~(2+),but in the case of pure MeOH is:Ni~(2+)>Cu~(2+)>Co~(2+)and this selectivity order is changed with the composition of the mixed solvents.The values of thermodynamic parameters(DELTA H°c,DELTA S°c)for comple-xation reactions were obtained from temperature dependence of the stability constants and the results show that in most cases,the complexes are enthalpy stabilized,but entropy destabilized and the values of DELTA H°c and DELTA S°c depend strongly on the nature of the medium.A non-linear behaviour is observed between the thermodynamic parameters and the composition of the mixed solvents.