MIXED-LIGAND COMPLEXES OF NICKEL(II) INVOLVING SULFUR-CONTAINING LIGANDS AND DIAMINOCARBOXYLIC ACIDS

Stability constants for mixed-ligand complexes of the types [NiABH(2)] , [NiABH] and [NiAB] formed by Ni-II with L-cysteine (cys), D-penicill amine (pen) or L-cysteic acid (cya) as ligand A and DL-2,3-diaminoprop ionic acid (dapa), DL-2,4-diaminobutyric acid (daba) or DL-ornithine ( orn) as ligand B have been determined by the computer-based analysis o f pH titration data obtained at 37 degrees C and I = 0.15 mol dm(-3)(N aClO4). In the [NiABH] species, for all three secondary ligands (B), w hen A = pen or cya the labile proton appears to be attached to the ter minal amino group of ligand B, whereas when A = cys it is not clear wh ere the proton is located. In all the systems in the [NiABH(2)] specie s, one proton resides with the primary ligand (A) and the other with t he secondary ligand (B). In the [NiAB]-type complexes, cys and pen che late through the amino and thiolato groups, while cya binds in a glyci ne-like mode and the secondary ligands (B) coordinate in a terdentate manner.