Studies on Nickel(II)-Pyridoxamine-Imidazole Containing Mixed Ligand Complex Systems

The stability constants of species present in the systems Ni(II)-pyridoxamine(pym)(A) and Ni(II)-pyridoxamine(pym)(A)-imidazole containing ligands(B) [B = imidazole(him),  benzimidazole(bim), histamine(hist) and L-histidine(his)] have been determined pH-metrically using the MINIQUAD computer program. The existence of the species NiAH, NiA and NiA2 was proven for the Ni(II)-pym(A) system, whereas for the Ni(II)-pym(A)-B systems NiABH2, NiABH, NiAB and NiAB2 species were identified. The pym ligand(A) binds the metal ion via its phenolic oxygen and amino methyl nitrogen atoms in the NiAB and NiAB2 species, and the binding of ligand B in these complexes is similar to their binding in the respective binary species. The site of protonation in NiABH and NiABH2 species is explained. A more positive D log K values for NiAB compared to NiAB2 species is discussed. The computer simulated distribution of the complexes in solution has been evaluated. The mixed ligand pyridoxamine complex of Ni(II) with L-histidine was prepared and characterized by conventional methods.