The reaction of o-phenylenediamine (OPD), sodium naphthalene1,5-disulfonate (Na₂NDS) and nickel sulfate in an ethanol–water mixture yielded the title compound, [Ni(OPD)₂(H₂O)₂]·NDS or [Ni(C₆H₈N₂)₂(H₂O)₂](C₁₀H₆O₆S₂). This salt consists of a complex [Ni(OPD)₂(H₂O)₂]²⁺ cation with two bidentate OPD ligands and trans aqua ligands, and a non-coordinating NDS^2– anion, which is the double-deprotonated form of H₂NDS. The Ni^II atom is situated at a center of inversion and exhibits a slightly tetragonally distorted {O₂N₄} octahedral coordination environment, with four shorter equatorial Ni—N bonds [2.0775 (17) and 2.0924 (18) Å] and a longer axial Ni—O bond [2.1381 (17) Å]. The OPD ligand is located about an inversion center and is nearly coplanar with the NiN₄ plane [dihedral angle 0.95 (9)°]. In the crystal, the cations and anions are connected by charge-assisted intermolecular N—H...O and O—H...O hydrogen-bonding interactions into the tri-periodic network structure. Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H...H (44.1%), O...H/H...O (34.3%), C...H/H...C (14.8%) C...C (6.5%) (involving the cations) and O...H/H...O (50%), H...H (25%), C...H/H...C (15.3%), C...C (8.2%) (involving the anions) interactions.

The title compound, [Ni(H₂O)₆](PHB)₂·2H₂O (1) (PHB = 4-hydroxybenzoate, C₇H₅O₃), was obtained by the reaction of NiCl₂, 4-hydroxybenzoic acid (PHBA) and monoethanolamine in aqueous ethanol solution. The Ni^II ion is coordinated by six water molecules and is located on an inversion center. The outer coordination sphere in the asymmetric unit comprises one PHB anion and one water molecule, i.e. the compound is a salt and a hydrate consisting of three components. In the crystal, the components are packed into an intermolecular network stabilized by O—H...O hydrogen bonds. A comprehensive Hirshfeld surface analysis was carried out to investigate the intermolecular interactions in detail.