This study reports on the synthesis, crystal structure, and Hirshfeld surface investigation of the nickel(II) complex [Ni(C₆H₂N₂)(H₂O)₄]SO₄. NiSO₄·6H₂O and 8-aminoquinoline were reacted in ethanol under reflux conditions at 60 °C for 30 minutes to create the complex. Slow evaporation of the reaction solution at room temperature yielded light-green single crystals suitable for X-ray diffraction investigation (75% yield). The results of the elemental analysis are in good agreement with the proposed molecular formula [Ni(C₉H₈N₂)(H₂O)₄]SO₄. The compound crystallizes in the monoclinic crystal system with the space group P21/c, according to single-crystal X-ray diffraction study. The unit-cell volume is 1473.33(14) ų, Z = 4, a = 12.0672(6) Å, b = 9.2001(4) Å, c = 14.4120(8) Å, and β = 112.953°.

The nickel (II) ion possesses a six-coordination number and a deformed octahedral shape generated by one nitrogen atom from the 8-aminoquinoline ligand and four coordinated water molecules, according to structural analysis. The Ni-N and Ni-O bonds measure 2.106 Å and 2.081(3) Å, respectively.
A large hydrogen-bonding network including coordinated water molecules, the ligand's amino group, and the sulfate anion helps to maintain the crystal packing. The cationic complex and sulfate anion form layered supramolecular structures through twelve hydrogen bonds of the N-H···O and O-H···O kinds, with sulfate oxygen atoms acting as acceptors.

Hirshfeld surface analysis was used to explore intermolecular interactions in the crystal structure. The findings show that H···O/O·· ·H interactions account for 45.0% of total contacts in crystal packing, followed by H···H (34.4%) and H···C/C···H (14.3%) interactions. Hydrogen bonding plays a crucial role in crystal structure stabilization, as evidenced by minor contributions from C···C (4.3%) and H···N/N···H (0.8%) contacts, which, although small, indicate that these interactions still contribute to the overall stability of the crystal structure.