Formation of cobalt(II) complexes with five pyridine oximes in aqueous solution

Complex formation equilibria of cobalt(II) with pyridine-2-carboxamidoxime (1), pyridine-2-acetamidoxime (2), pyridine-2-aldoxime (3), 1-(2-pyridinyl)ethanone oxime (4) and 6-methyl-pyridine-2-aldoxime (5) were studied in 0.1 M NaCl solution at 25 degrees C by potentiometric titrations with use of 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 1-4 Co(II) forms mono, bis and (except for 2) tris complexes of the type Co(HL)(r)(2+) and deprotonated/hydrolyzed products of the bis and tris complexes, Co(HL)L+ and Co(HL)(2)L+. The deprotonated complexes have a low spin structure (t(2g) (6)e(g)) and are readily oxidized after the easy loss of the sole e(g) electron. Their formation with increasing pH involves slow attainment of equilibrium in the pH range 3-5. Only with ligand 3 could studies be continued to pH 8-10 by using very low cobalt(II) ion concentrations. There, CoL2 is formed quantitatively, while complexes Co(HL)L-2 and Col(3)(_) could not be observed. Ligand I also forms the complex Co-2(HL)(2)H2L5+ and ligand 2 the complex Co(H2L)(3+) with the positively charged ligand (H2L+). Ligand 5 forms the complexes Co(HL)(2+), CoL2, Co2L2OH+, Co2L3+ and Co2L3OH mainly in the pH range 6-10. The stabilities of these complexes and also their oxidation reactions are reduced by the steric requirements of the 6-methyl groups of the ligand.