Stoichiometry and stability of complexes formed between 18-crown-6 as wellas dibenzo-18-crown-6 ligands and a few metal ions in some non-aqueous binary systems using square wave polarography

The complexation reaction between Pb2+, TI+ and Cd2+ ions and macrocyclic l igands, 18-crown-6 (18C6) and dibenzo-18-crown-6 (DB18C6), was studied in d imethylsulfoxide (DMSO)-nitromethane (NM) and dimethylformamide (DMF)-nitro methane binary systems by square wave polarography (SWP) technique. The sto ichiometry and stability of the complexes were determined by monitoring the shifts in half-waves or peak potentials of the polarographic waves of meta l ions against the ligand concentration. In most cases, the stability const ants of complexes increase with increasing amounts of the nitromethane in m ixed binary solvents used in this study. The complexes formed between 18C6 and DB 18C6 and these metal cations in all cases had a stoichiometry of 1 : 1. The results obtained show that there is an inverse relationship between the formation constant of complexes and the donor number of solvents based on a Gatmann donocity scale and the stability constants show a high sensit ivity to the composition of the mixed solvent systems. A linear behavior wa s observed for variation of log K-f of 18C6 complexes vs the composition of the mixed solvent systems in NM/DMSO and NM/DMF, but a non-linear behavior was observed in the case of DB 18C6 complexes in these binary systems. In most of the systems investigated, the Pb2+ cation forms a more stable compl ex with the 18C6 than other two cations and the order of selectivity of thi s ligand for cations is: Pb2+ > T1(+), Cd2+.