Comparative Stability of mixed ligand complexes of Ni(II) and Zn(II) metal ions with 2,6-Pyridinedicaboxylic acid (DPA) and Oxalic acid, Ethylenediamine, Glycine, Phenylalanine. Tyrosine

Abstract

By using SCOG (stability constant of genaralised species) computer program we can calculated the value of ∆logK.. If the value of ∆logK is more positive then the stability of the complex increases. The formation constant of ternary complexes [MAL], where M = Ni(II), Zn(II); A = 2,6-Pyridinedicaboxylic acid (DPA) and L = Oxalic acid, Ethylenediamine, Glycine, Phenylalanine, Tyrosine have been determined. The analysis of the representative species distribution curves show that the pH range 5-7 [ML] and [MAL] are the major species. The percentage of all other species is less than 1% in care of [M(DPA)(L)] system. This is because DPA forms stable binary complex M-DPA(MA) at low pH and L combines with [MA] to form [MAL] ternary complex. The ∆logK value becomes more negative with increasing the charge on the ligand L, e.g. oxalate> glycinate> ethylenediamine. The electronic repulsion is more with the increasing charge on Ln-, resulting in more negative ∆logK values. It is observed that for the complex [Ni(DPA)L] and [Zn(DPA)L] where L= ph-alanine and tyrocin, D logK value is less negative or positove than the complexes where L=ox, gly, a-ala. This is because of the intramolecular interligand interaction. More positive values of D logK for yhe systems with L= tyrosine because of hydrogen bonding between the phenolic –OH of the side group of tyrosine and the carboxylate (-COO-) group of the DPA. Due to the large size of Zn metal ion it can bind firmly the bulky ligand thereby increases the stability of Zn (II) over the stability of Ni (II) complexes. So Zn(II) complexes is more stable compare to Ni(II) complexes.