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Computer-based analysis of the pH-titration data obtained in aqueous perchlorate medium at 37 "C and / = 0.1 5 mol dm-3 (NaCIO,) for Nil1-A-B ternary systems [A = L-cysteine (Cys), o-penicillamine (pen) or L-cysteic acid (Cya); B = imidazole (Him), histamine (Hist) or L-histidine (His)] showed the presence of the ternary species NiABH,, NiABH, NiAB and NiAB, in addition to binary species with A or B. Under the same experimental conditions, binary systems of Nil' with Cys, pen, Cya, Him, Hist and His have also been studied. For the NiABH species in the Ni"-Cys-Him, -His and -Hist and Nil'-pen-Him mixed-ligand systems, the site of protonation is on the primary ligand (A), while in the Nil1-A-His and -Hist systems (A = pen or Cya) protonation occurs at the secondary ligand. In the NiABH, species, one proton is attached to the primary ligand and the other to the secondary ligand. For the NiAB and NiAB, species the Cys and pen primary ligands bind the metal through the mercapto and amino groups, while Cya binds Nil1 in a glycine-like mode. In these species, the Him, Hist and His secondary ligands are respectively mono-, biand ter-dentate. The probable reason for the higher stability of NiAB, species compared to NiAB species in the Nil1-A-Him systems (A = Cys, pen or Cya) is discussed.

Stability constants for mixed-ligand complexes of the types [NiABH2], [NiABH] and [NiAB] formed by Ni n with L-cysteine (cys), D-penicillamine (pen) or L-cysteic acid (cya) as ligand A and DL-2,3-diaminopropionic acid (dapa), DL-2,4-diaminobutyric acid (daba) or DL-ornithine (orn) as ligand B have been determined by the computerbased analysis of pH titration data obtained at 37 ~ and I = 0.15 tool dm-3 (NaC10~). In the [NiABH] species, for all three secondary ligands (B), when A = pen or cya the labile proton appears to be attached to the terminal amino group of ligand B, whereas when A = cys it is not clear where the proton is located. In all the systems in the [NiABH2] species, one proton resides with the primary ligand (A) and the other with the secondary ligand (B). In the [NiAB]-type complexes, cys and pen chelate through the amino and thiolato groups, while cya binds in a glycine-like mode and the secondary ligands (B) coordinate in a terdentate manner.