Complexation of cobalt(II), nickel(II), and zinc(II) ions with mono and binucleating azo compounds: A potentiometric and spectroscopic study in aqueous solution

The interaction of Co(II), Ni(II) and Zn(II) ions with a series of aromatic alpha-hydroxy azo compounds, 1,4-bis-p-sulfonylazo-2,3-dihydroxy naphthale ne: 4-(2-hydroxy-1-phenylazo)-benzenesulfonate, 4-(2-hydroxy-1-naphthylazo) -benzenesulfonate, and 4-(9-hydroxy-10-phenanthrylazo)-benzenesulfonate, ha s been studied in aqueous solution. While both mono and binuclear chelates were found in the complex systems formed by the first azo compound, the exi stence of mononuclear chelates with different degrees of protonation was ve rified in the systems formed by the other ones. The equilibrium constants f or all metal complexes were evaluated using a combination of potentiometric and spectrophotometric titration data. Within the mononucleating ligands, the magnitudes of the complexes equilibrium constants were to be found simi lar, even when their aromaticities are different. Both the metal coordinati on sites and the tautomeric structures of the ligand in each complex have b een inferred through spectroscopic analysis. (C) 1998 Elsevier Science Ltd. All rights reserved.