Metal Complexes of some Peptide Derivatives. Part-VIII. Equilibrium Study on the Complex Formation of Cobalt(II), Nickel(II) and Zinc(II) with some Salicyloylamino Acids

Department of Chemistry, University College of Science, 92 Acharya Prafulla Chandra Road, Calcutta-700 009

Manuscript received 17 May 1988, revised 11 January 1989, accepted 23 February 1989

pH-metric study on the complexation equilibria of Co^II, Ni^II and Zn^II (M²⁺⁾ with some salicyloylamino acids, (o)HO C₆H₄C(-O)NHCH(R)COOH) or (H₂L), where R=H (salicyloyl glycine), CH₂ (salicyloyl dl-α-alanine), CH₂CH₂SCH₂ (sail­cyloyl dl-methionine), provided evidence of formation of ML, \(ML^{2-}_2\)^- and ML(OH)^­complexes through bidentate (O,\(\bar{O}\)) chelation by the amide carbonyl oxygen atom and phenolate oxygen atom of the ligand, leaving the carboxylate group uncoordinated in these complexes. Formation constants at 25±0.1° in 50% (v/v) aqueous ethanol medium and ionic strength µ = 0 2 M (NaNO₂) were in the order. Co^II < Ni^II < zn^II for the ML and \(ML^{2-}_2\) complexes, in the order Co^II > Ni^II > Zn^II for the M(OH) (L)^- complexes with all the three ligands, and in the order, salicyloylglycine > salicyloyl-dI- \(\propto\)-atanine > salicyloyl dl-methionine for all the three metal ions. The results have been correlated with the modes of bonding of the ligands in the resulting complexes and compared with the corresponding values of the complexes formed by Cu^II.