Binary and ternary complexes of chromium(III) involving iminodiacetic acid, L(+)-aspartic acid, L(+)-glutamic acid, or L(+)-cysteine as ligands

Abstract

Stability constants for binary complexes of the type MAH, MA, MA₂H₂, MA₂H,MA₂, or M₂A₃ in the CrIII-iminodiacetic acid (H₂ida), -L(+)-aspartic acid (asp), -L(+)-glutamic acid (glu), and -L(+)-cysteine (cys) systems and for ternary complexes MABH₂, MABH, or MAB in the Cr^III-H₂ida(A)-asp or -glu(B) and Cr^III-cys(A)-asp or -glu(B) systems have been determined for the first time at 50±0.1°C and l=0.1 mol dm⁻³(Na[ClO₄]) by potentiometry using a computer iteration technique. The acid dissociation constants of these ligands have also been obtained under identical conditions. The pH dependence of the electronic spectra of the equilibrated solutions provides information on the nature and type of complexes formed. The results suggest that H₂ida and asp act as tridentate ligands in the binary complexes MA and MA₂, as well as in the ternary complex MAB. However, it appears that glu is tridentate in the MA complex, but bidentate in MA₂ and MAB. The potentially tridentate ligand cys has been established to behave as a bidentate ligand towards Cr^III. The probable site of protonation in the binary complexes MAH, MA₂H, and MA₂H₂ is discussed on the basis of the dissociation constant of the ligand and the stability-constant data for the complexes. Preferential formation and enhanced stability were observed for the ternary relative to the binary complexes. This has revealed factors such as ring size, ligand-ligand interactions, and steric effects which influence the stabilization of chromium(III) mixed-ligand complexes.