A Conductometric Study of Complexation Reactions Between 4-Nitrobenzo-15-Crown-5 with Ag+, Hg2+, Tl+ and La3+ Metal Ions in Acetonitrile-Methanol Binary Solutions

Abstract

The complex formation between Ag⁺, Hg²⁺, Tl⁺ and La³⁺   metal ions with macrocyclic ligand, 4-nitrobenzo-15-crown-5   (NB15C5) was studied in acetonitrile (AN)-methanol (MeOH)   binary systems at different temperatures using conductometric   method. The obtained results show that the stoichiometry of   the complexes is 1:1 (ML), but in the case of Hg²⁺ cation, in   addition to a 1:1 complex, a 1:2 (ML₂) complex is formed in   solutions. The stability constants of the complexes were   obtained from fitting of molar conductivity curves using a   computer program, Genplot. A non-linear behaviour was   observed for changes of log K_f of the complexes versus the   composition of AN-MeOH binary solution. The results show   that the selectivity of NB15C5 for the metal cations changes   with the composition of the mixed solvent. The values of   standard enthalpy changes (ΔH°_c) for complexation reactions   were obtained from the slope of the van't Hoff plots and the   changes in standard entropy (ΔS°_c) were calculated from the   relationship: ΔG°^{c, 298.15} = ΔH°_c - 298.15 ΔS°_c. The obtained   results show that the complexes are entropy stabilized but   from the enthalpy view point, depending on the solvent system   the metal ion complexes are stabilized or destabilized and the   values of ΔH°_c and ΔS°_c depend strongly on the nature of the   medium.