TERNARY COMPLEXES OF SOME TRANSITION METALS WITH TETRADENTATE MERCAPTOPROPIONAMIDES

The stability constants of ternary cu(ii), zn(ii), cd(ii), co(ii) and ni(ii) complexes containing 2,2'- bipyridyl (bipy.) or 1,10-phenanthroline (phen.) as primary ligand and n,n'-dimercaptopropionyl ethylene-diamine (dmped), n,n'-dimercaptopionylpropylene diamine (dmppd) or n,n'- dimercaptopropionyl-butylene diamine (dmpbd) as secondary ligand were determined in aqueous medium at 30 ± 1 °c and 0.1m, (naclo4) ionic strength in a nitrogen atmosphere. a study of the title metal ions in the presence of an equimolar concentration of the two ligands showed the formation of mixed-ligand complexes containing 1:1:1 molar ratio of metal ion and the two ligands. these ternary complexes are found more stable than would be expected on statistical reasons, i.e., the difference ∆ log k m= log kma mal - log k ma malis positive (a = bipy or phen, l = dmped, dmppd or dmpbd) this increased stability is discussed on the basis of π-acceptor qualities of hetero aromatic n-base and the occurrence of a cooperative effect due to π-systems of two ligands bound with the same metal ions. the ternary complexes formed with phen. as ligand are less favoured than the corresponding complexes formed with bipy. and is discussed on the basis of steric reasons.