Formation of zinc(II) and cadmium(II) complexes with pyridine oxime ligands in aqueous solution

Complex formation equilibria involving pyridine-2-carboxaldehyde oxime (1). 1-(2-pyridinyl)ethanone oxime (2) and 6-methylpyridine-2-carboxaldehyde oxime (3), HL, with zinc(II) and cadmium(11) ions were studied in aqueous 0.1 M NaCl solution at 25degrees C by potentiometric titrations with a glass electrode. Experimental data were analysed with the least-squares computer program SUPERQUAD to determine the complexes formed and their stability constants. With Ligands I and 2 the sets of complexes for Zn(II) and Cd(II) are essentially the same, mono- and dinuclear oxime complexes and their deprotonated/hydrolysed products HpMq(HL)(r)(2q+p). Owing to the steric requirements of the 6-methyl group, sets of complexes formed with 3 are distinctly different. For zinc(II), only dinuclear oximato species H(p)Z(n2)(HL)(2)(4+p) (p = - 2, - 3 - 4) are found, while for the larger cadmium(II) ion mononuclear oximato species CdL4 and CdL2 are detected in addition to the dinuclear complex HpCd2(HL)(2)(4+p) (p =-3).