Potentiometric studies, synthesis and characterization of mixed-ligand complexes of copper(II), nickel(II), cobalt(II) and manganese(II) with N-(2-acetamidoiminodiacetic) acid as the primary ligand and histidine as the secondary one

The formation equilibria of new ternary mixed-ligand complexes of copper(II ), nickel(II), cobalt(II), and manganese(II) with N-(2-acetamidoiminodiacet ic) acid (H(2)ADA) as the primary ligand and histidine (His) as the seconda ry one were investigated. The formation constants of these complexes have b een determined at 25.0 degreesC in 0.3 M aqueous Na2SO4 solution. They show that the ternary complexes are more stable than the corresponding binary o nes. The ternary complexes have been synthesized and were characterized usi ng elemental analyses, molar conductance, IR spectra, magnetic measurements as well as electronic and ESR spectra. The spectral results support the bi nding of H(2)ADA and histidine with, respectively, the NOO and NO sites, to copper(II), cobalt(II), and manganese(II) providing an octahedral geometry , while nickel(II) is bonded to the same ligands in a bidentate manner by t he OO and NO donor groups giving a square-pyramidal complex. The other axia l sites are occupied by OH- or H2O groups. Magnetic and ESR data indicate t hat cobalt(II) and manganese(II) adopt a high-spin configuration in the com plexes and also show that the metal ions do not undergo any change in the o xidation state.