Equilibrium and structural studies on metal complexes of oxime ligands. Formation of copper(II) and nickel(II) complexes of pyridine-2-carboxamidoxime and pyridine-2-acetamidoxime in aqueous solution

The protonation and complex formation equilibria of pyridine-2-carboxamidoxime (HL=1) and pyridine-2-acetamidoxime (HL=2) with copper(II) and nickel(II) ions were studied at 25 degrees C in aqueous 0.1 mol dm(-3) NaCl solution by potentiometric titration. Experimental data were analysed with respect to the complexes formed and their stability constants, by using the least-squares computer program SUPERQUAD. In the ligand 1 systems the data for copper(II) can be explained in terms of the mononuclear binary complexes Cu(HL)(r)(2+) (r=1, 2) and Cu(HL)L(+), and the data for nickel(II) in terms of the stepwise binary complexes Ni(HL)(r)(2+) (r=1, 2, 3) together with a series of hydrolysed species HpNi(HL)(r)(2+p), where r=2, 3 and -p=1, 2, 3. With ligand 2 the best model for copper(II) was that including the mononuclear complexes Cu(HL)(r)(2+) (r=1,2) and the dimeric species H-2Cu2(HL)(2)(2+), and the best model for nickel(II) consisted of the mononuclear complexes Ni(HL)(r)(2+) (r= 1, 2) and Ni(HL)L(+) together with the dimeric species H-3Ni2(HL)(2)(+). The effect of py substituent in amidoxime compounds was studied by comparing the complex models with those reported earlier for similar aminoamidoxime systems, and it is concluded that the amide group of these py-substituted ligands does not take part in the coordination to the metal.