Stability and structure of some binary and mixed-ligand complexes of zinc(II)

Abstract

The stability and structure of ZnBH, ZnB, ZnB₂H₂, ZnB₂H, or ZnB₂ types of binary complexes in the Zn^II–DL-2,3-diaminopropionic acid (dapa), –DL-2,4-diaminobutyric acid (daba), and –DL- ornithine (Orn)(B); and ZnABH₂, ZnABH, or ZnAB types of mixed-ligand complexes in the Zn^II–histamine/L-histidine (A)–dapa,–daba, and –Orn (B) systems are discussed from the computer based analysis of the pH titration data at 37 °C and I= 0.15 mol dm^–3(NaClO₄). In both the binary and mixed-ligand systems, dapa, daba, and Orn (B) appear to be tridentate. The results clearly demonstrate that in the ZnBH, ZnB₂H₂, and ZnB₂H binary complexes and ZnABH mixed species the site of protonation is the terminal amino- group of ligand B. In the ZnABH₂ complexes, one proton is attached to the ligand A and other to the ligand B. In the Zn^II–histidine (A)–dapa,–daba, and –Orn (B) systems, the results suggest the histamine- like mode of binding for L-histidine (A). More than the statistical order of stabilities was observed for most of the mixed-ligand complex species detected in the present investigation.