Abstract
The stability constants of ternary Zn(II), Cd(II), Co(II) or Ni(II) complexes containing 2,2′‐bipyridinyl (Bipy) or 1,10‐phenanthroline (Phen) and N‐(thiobenzoyl)‐N‐phenylhydroxylamine (TBPH) have been determined in 70% (v/v) dioxane/water medium at 30 ± 1°C and at constant ionic strength (0.1 M NaClO~4~) in a nitrogen atmosphere. A study of the title metal ions in the presence of an equimolar concentration of the two ligands revealed the formation of mixed‐ligand complexes containing a 1:1:1 molar ratio of metal and the two ligands. These ternary complexes are more stable than would be expected on a statistical basis, i.e. the difference, Δlog K~M~ = log K^ma^~mab~ ‐ log K^m^~mb~, is positive. This is discussed on the basis of π‐acceptor qualities of the heteroaromatic N‐base and the occurrence of a co‐operative effect due to the π systems of two ligands bound to the same metal ion. The trend in stability constants of the mixed‐ligand complexes of the title metal ions is found to be Ni > Co > Cd > Zn. Mixed‐ligand complexes formed with 1,10–phenanthroline are found to be less stable than those formed with 2,2′‐bipyridine and this is discussed on the basis of steric factors.