Stability constants for the mixed ligand species of the types NiAB, NiABH and NiABH(2) (where A=dopamine and dopa; B=L-cysteine, D-penicilamine and L-cysteic acid) have been determined. In the NiAB, the mode of coordination of A and B is exactly similar to their binding in the respective binary complexes. In NiABH, the extra proton is attached to A. In NiABH(2) (where A=dopamine) one proton is attached to A and other resides with B, while in the NiABH(2) (where A=dopa) both the protons are with dopa.
M. S. Nair, Dr. Asha Sathish, and Pillai, M. S., “Studies on the stabilities of some mixed ligand complexes of nickel (II) involving dopamine/dopa and sulphur containing ligands”, Indian Journal of Chemistry, 1999.