2,9-Dimethyl-1,10-phenanthrolin-1-ium (6-carb-oxy-4-hy-droxy-pyridine-2-carboxyl-ato-κO,N,O)(4-hy-droxy-pyridine-2,6-dicarboxyl-ato-κO,N,O)nickelate(II) 2.35-hydrate: a proton-transfer compound.

The title proton-transfer compound, (C(14)H(13)N(2))[Ni(C(7)H(3)NO(5))(C(7)H(4)NO(5))]·2.35H(2)O, consists of an [Ni(hypydc)(hypydcH)](-) anion, a dmpH(+) cation and 2.35 uncoordinated water mol-ecules (where hypydcH(2) = 4-hy-droxy-pyridine-2,6-dicarb-oxy-lic acid and dmp = 2,9-dimethyl-1,10-phenanthroline). The Ni(II) atom is coordinated by two N atoms and four O atoms from the carboxyl-ate groups of the (hypydc)(2-) and (hypydcH)(-) ligands, forming a distorted octa-hedral environment. In the anion, the two pyridine rings are inclined to one another by 89.24 (10)°. In the crystal, cations are linked via O-H⋯O hydrogen bonds forming dimers, graph-set [R(2) (2)(16)], centered about inversion centers. These dimers are further linked by other cation O-H⋯O hydrogen bonds, graph-set [R(6) (6)(42)], forming a two-dimensional network in (011). Additional inter-molecular O-H⋯O, N-H⋯O, N-H⋯N, and weak C-H⋯O hydrogen bonds, and π-π inter-actions [shortest centroid-centroid distance = 3.5442 (14) Å], connect the two dimensional networks, forming a three-dimensional arrangement. The H atoms of one of the methyl groups are disordered over two sites with equal occupancy.