Solution equilibria and stabilities of binary and ternary Nickel(II) complexes with picolinic acid and small blood serum bioligands

Abstract

The binary and ternary complexes of Nickel(II) with picolinic acid (Hpic) as a primary ligand and the small blood serum bioligands lactic acid (Hlac), oxalic acid (H2ox), citric acid (H3cit) and phosphoric acid (H3PO4) as biologically relevant secondary ligands were studied. The acidity constants of the ligands selected were determined potentiometrically and used for determining the stability constants of the complexes formed in aqueous solution at 25°C and using an ionic strength I=1.0moldm−3 (NaCl). The analysis of the potentiometric data by means of the computational least-squares program LETAGROP indicates the Ni(II)–Hlac system in the formation of the complexes [NiL]+ and [NiL(OH)2]−. The Ni(II)–H3cit system indicates the formation of the species [NiL]−, [Ni(HL)L]3− and [NiL2]4−. In the Ni(II)–H3PO4 system the complexes [NiH2L]+, NiHL and [NiL]− were detected. In the case of the ternary systems we observed in the Nickel(II)–Hpic–Hlac system the formation of the complexes Ni(pic)(L), [Ni(pic)(L)(OH)]− and [Ni(pic)(L)(OH)2]2−. In the Nickel(II)–Hpic–H2ox system the species [Ni(pic)(L)]−, [Ni(pic)(L)(OH)]2− and [Ni(pic)2(L)]2− were observed. In the Nickel(II)–Hpic–H3cit system the complexes [Ni(pic)(HL)]−, [Ni(pic)(L)]2− and [Ni(pic)(L)(OH)]3− were detected. Finally, in the Nickel(II)–Hpic–H3PO4 system the complexes Ni(pic)(H2L), [Ni(pic)(HL)]− and [Ni(pic)(L)]2− were observed. Species distribution diagrams as a function of pH were briefly discussed.