Solution behaviour of mixed ligand complexes of nickel(II) involving penicillin group drugs and sulfur containing ligands under physiological conditions

Analysis of the pH-titration data obtained in aqueous perchlorate medium at 37 degrees and I = 0.15 mol dm(-3) (NaClO4) for Ni-II-A-B mixed ligand sys tems [A = 6apa or amp; B = cys, smc, pen or cya] shows the presence of mixe d ligand complex species NiABH, NiAB and/or NiAH(-1)B in addition to variou s binary species due to ligands A and B. The pK(NiABII)(II) values obtained in all these systems are close to each other, indicating the site of proto nation in the NiABH species to be with ligand A. In the NiAB type of comple xes, coordination of the ligand A results in the formation of a five-member ed ring. In the NiAH(-1)B species in the Ni-II-amp(A)-B systems, deprotonat ed AH(-1) ligand binds the metal in a tridentate manner. In all the mixed l igand complexes, cys, smc, pen and cya(B) ligands are bidentate. The statis tical parameter Delta log K calculated indicates preferential formation and enhanced stability for the mixed ligand complexes relative to the binary o ne.