Complexation Equilibria and Determination of Stability Constants of Binary and Ternary Nickel(II) Complexes with Amino Acids (Glycine, α-Alanine, β-Alanine and Proline) and Dipicolinic Acid as Ligands

Abstract

In this paper we report the formation of binary and ternary nickel(II) complexes involving dipicolinic acid (H2Dipic) as the primary ligand and some selected amino acids {glycine (HGly), ⍺-alanine (H⍺-Ala), β-alanine (Hβ-Ala) and proline (HPro)} as secondary ligands. These complexes were studied at 25 °C by means of electromotive force measurements, emf(H), using 1.0 mol⋅dm−3 NaCl as the ionic medium. The experimental data were analyzed by means of the computational least-squares program LETAGROP, taking into account hydrolysis of the nickel(II) cation and the acid/base reactions of the ligands whose equilibrium constants were kept fixed during the analysis. In the study of the binary nickel(II)–amino acids systems the species [NiL]+, NiL2 and [NiL3]− were observed, and in the case of the ternary nickel(II)–dipicolinic acid–amino acids systems the complexes Ni(Dipic)HL, [Ni(Dipic)L]− and [Ni(Dipic)L(OH)]2− were observed. The respective stability constants were determined, and the species distribution diagrams, as a function of pH, are briefly discussed.