A Potentiometric Study on Mixed Ligand Cadmium(II) Complexes with 2-Amino Carboxylic Acids

Abstract

Abstract Formation constants of mixed ligand cadmium(II) complexes, CdLL′, were determined in aqueous solutions containing 3 mol dm−3 LiClO4 as a constant ionic medium at 25 °C by potentiometric titration, where the ligands HL and HL′ are 2-amino carboxylic acids such as glycine, α-alanine, 2-aminobutanoic acid, 2-aminopentanoic acid, and 2-aminohexanoic acid. Ten kinds of the mixed ligand cadmium(II) complexes were examined with the combination of five 2-amino carboxylic acids. The formation constant, βprs=[CdpLrLs′(2p−r−s)+]⁄([Cd2+]p×[L−]r[L′−]s), of the mixed ligand cadmium(II) complexes containing glycine as one of the ligands lie between those of the parent complexes, while the formation constants of the other complexes are larger than those of the parent complexes. No protonated mixed ligand complex has been observed. The small values of β111 of the Cd(gly)L complexes are due to the considerably small values of the first stepwise formation constants of the CdL+ complexes. On the other hand, the stepwise formation constants of the CdL+ complexes with glycinate ion are larger than the stepwise formation constants with 2-amino carboxylate ions and the second stepwise constant of the bis(glycinato)cadmium(II) complex. Interactions between the ligands through the central cadmium(II) ion are discussed. The b-values (b=log {(K110·K101)⁄(K120·K102)}, where K1rs denotes the stepwise formation constant of the complex CdLrL′s) of the mixed ligand cadmium(II) complexes containing glycinate ion as one of the components lie between 0.93 and 0.98, and are larger than those of the other complexes (b=0.83–0.90), the fact indicating that the mixed ligand glycinato complexes have larger inter-ligand interactions within the complexes than those within complexes containing other 2-amino carboxylate ions. The inter-ligand interaction may occur through the central cadmium ion within the complex.