The new compounds, (phhyH)2(pydc), 1 and (phhyH)[Ni(pydc)(pydcH)].3H2O, 2, phhy: phenylhydrazine, pydcH2: pyridine- 2,6-dicarboxylic acid, were synthesized and characterized by IR, 1H NMR, 13C NMR and UV spectroscopy. The structure of 2 was determined by X-ray crystallography. In the crystal structure, the metal ion was six coordinated by two tridentate (pydc)2- and (pydcH)- groups and each anionic complex unit, [Ni(pydc)(pydcH)]- was accompanied by one (phhyH)+ as counter ion and two water molecules. In compound 2, a large number of O-H···O, N-H···O and C-H···O hydrogen bonds were observed. These interactions as well as other noncovalent interactions such as ion pairing, C-O··· and - stacking play an important role in the formation and stabilization of the crystal lattice. In the solution study, the equilibrium constants for the binary pydc-phhy proton transfer system, the stoichiometry and stability of complexation of this system with Ni2+ ion in aqueous solution were investigated using potentiometric pH titration method. The stoichiometry of the most abundant species in the solution was compared to the crystalline cited metal ion complexes.
