STUDIES ON BIOLOGICALLY RELEVANT TERNARY METAL-COMPLEXES .6. STABILITY OF TERNARY CO(II), NI(II), CU(II) AND ZN(II) COMPLEXES INVOLVING AMINOPOLYCARBOXYLIC ACIDS AND AMINO-ACIDS

Ternary metal complexes of the type MLA, where M = Co(II), Ni(II), Cu(II) and Zn(II); L = iminodiacetic acid (IMDA) or nitrilotriacetic acid (NTA) and A = glycine, alanine, valine, leucine, norleucine, phenylalanine, tryptophan, serine, threonine, methionine, aspartic acid, ethylenediamine or pyrocatechol have been investigated potentiometrically at 35-degrees-C and mu = 0.2 M (KNO3). The stabilities of the ternary complexes have been quantitatively compared with the stabilities of the corresponding binary metal complexes (MA) determined under identical experimental conditions. Ternary complexes containing IMDA are found to be more stable than the corresponding complexes containing NTA. With respect to ligand L, the stability of ternary complexes increases in the order: pyrocatechol (-O - O- donor) < bidentate amino acid (N - O- donor) < ethylenediamine (N - N donor). Cu(II) ternary complexes are found to be less stable than the corresponding Co(II), Ni(II) or Zn(II) complexes. Ternary Cu(II) complexes containing bis(imidazol-2-yl)methane are more stable than the corresponding complexes containing IMDA or NTA. Various factors leading to differences in the relative stabilities of the ternary complexes are discussed.