COMPLEXES OF 3,4-DIHYDROXYPHENYL DERIVATIVES .3. EQUILIBRIUM STUDY OF PARENT AND SOME MIXED-LIGAND COMPLEXES OF DOPAMINE, ALANINE AND PYROCATECHOL WITH NICKEL(II), COPPER(II) AND ZINC(II) IONS

The equilibrium constant of the parent complexes of nickel(II), copper(II) and zinc(II) with dopamine, and of the mixed ligand complexes wiht alanine or pyrocatechol as ligand B, were determined pH-metrically at 25°C and 0.2 M ionic strength. As regards the parent complexes, it was found that only the ortho phenolic hydroxy groups take part in hte coordination. The 1:1 complexes MHA and MA, and the 1:2 complexes MH2A2, MHA2 and MA2 are formed. It was demostrated that the stability relations of the mixed complexes formed with pyrocatechol as ligand B correspond to the statistical case, as a consequence of the values of logK1/K2 of the parent complexes being almost the same. An increased stability compared to the statistical case was observed for mixed complexes with alanine. This was interpreted by the different stability relations of the parent complexes and by the role of neutralization of charge for the complexes of all three metal ions. © 1979.