Conductometric Studies of Solvent Influence upon Complex Formation Between 4?-Nitrobenzo-15-crown-5 and Ni2+, Co2+ and Cu2+ in Acetonitrile-Methanol Binary Mixtures

Abstract

The complex formation between Ni²⁺, Co²⁺ and Cu²⁺ metal   cations with 4'-nitrobenzo-15-crown-5 was studied in acetonitrile   (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: Co²⁺ > Cu²⁺ > Ni²⁺, but in the case of pure   MeOH is: Ni²⁺ > Cu²⁺ > Co²⁺ and this selectivity order is   changed with the composition of the mixed solvents. The   values of thermodynamic parameters (ΔHºc, ΔSºc) for complexation   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 ΔHºc and ΔSºc 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.