Mixed-ligand nickel(ii) and copper(ii) complexes in competitive chelation reactions with polyaminopolyacetate and polyamine ligands

The mixed-ligand and competitive formation of coordination compounds in aqueous solutions of ternary systems containing nickel(ii) or copper(ii) cations, polyaminopolyacetate (nitrilo triacetic, ethylenediaminetetraacetic, or diethylenetriaminepentaacetic acid), and polyamine (ethylenediamine, diethylenetriamine, or triethylenetetramine) ligands was studied by spectrophotometry. The factors were determined which facilitate the formation of mixed-ligand complexes of composition [ML1L2] (M is NiII or CuII, L1 is a polyaminopolyacetate chelating agent, L2 is a polyamine ligand) in aqueous solutions. In these complexes, amines act predominantly as polydentate chelating ligands and the coordination sphere of metal atoms is completed by nitrogen and oxygen atoms of the chelating agents. An increase in the denticity of the polyamine ligand was found to increase the number of ethylenediamine metallocycles and promote partial cleavage of glycinate rings of carboxylate ligands in the coordination polyhedron of the chelate, thereby providing an increase in the stability of the resulting mixed-ligand complex species. In systems containing copper(ii) cations, the ability of polyamine ligands to exhibit a discriminatory effect and block the coordination of the polyaminopolyacetate chelating ligand was shown to vary and depend on the combination of L1 and L2 ligands in the coordination sphere.