Ring size effects on the chemical properties of tervalent nickel complexes with tetra-aza macrocyclic ligands in aqueous solutions. An electrochemical and pulse radiolytic study

Electrochemical oxidation and oxidation by hydroxyl radicals of the divalent nickel complexes of 1,4,7,10-tetra-azacyclotetradecane and 1,4,8,12-tetra-azacyclopentadecane in acidic aqueous solutions containing perchlorate or sulphate anions are reported. The properties of the tervalent complexes thus formed are described. The results are compared with those reported earlier for other tervalent nickel complexes with tetra-aza macrocyclic ligands. The results indicate that neither the redox potentials nor the half-lives of the tervalent complexes are directly related to the cavity size of the ligand or its ligand-field strength. It is suggested that the detailed conformation of the macrocyclic ligands has a profound effect on the properties of the tervalent complexes. The difference in the factors affecting the chemical properties of the mono- and ter-valent complexes is discussed.