Mixed-ligand complexes of nickel(II) involving sulphur-containing ligands and diaminocarboxylic acids

Summary Stability constants for mixed-ligand complexes of the types NiABH₂], NiABH] and NiAB] formed by Ni^II 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 °C and I = 0.15 mol dm^–3 (NaClO₄). 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.Author to whom all correspondence should be directed.