The new 1,10-phenanthroline containing 1:1 proton-transfer compound LH(2), [pyda.H(2)](2+)[phendc](2-), was synthesized from the reaction of 2,6-pyridinediamine, pyda, and 1,10-phenanthroline-2,9-dicarboxylic acid, phendc.H(2), and characterized by elemental analysis, ES-Ms, IR, (1)H, (13)C NMR, and UV/vis spectroscopies. Subsequently, the first example of [phendc](2)(-) containing anionic complex [pyda.H](2)[Co(phendc)(2)].10H(2)O, was prepared, using the above novel proton-transfer compound, and structurally characterized by single-crystal X-ray diffraction. The complex crystallizes in the space group P2(1)/n of the monoclinic system with four molecules in a unit cell of dimensions a = 11.877(3) A, b = 31.473(9) A, c = 12.915(4) A, and beta = 116.223(5) degrees. The structure has been refined to a final value for the crystallographic R factor of 0.0524 based on 9021 observed independent reflections. The complexation reactions of pyda, phendc.H(2), and LH(2) with H(+) as well as LH(2) with Co(II) in aqueous solution were investigated by potentiometric pH titrations, and the equilibrium constants for all major complexes formed are described. The results are presented in the form of distribution diagrams revealing the concentrations of individual complex species as a function of pH. The results revealed that, at a pH range of 5.2-6.2, the major complex species is [(pyda.H)](2)[Co(phendc)(2)], similar to the isolated crystalline complex.