The mononuclear complex [Ni(HOphen)(OSO3)(H2O)3]·5H2O (HOphen = 1, 10-phenanthrolin-2-ol) was prepared and its single structure was determined by X-ray crystallography. In this complex, the NiII ion has a distorted octahedral arrangement. Crystal structure analysis shows that two kinds of π–π stacking interactions and C–H···O short contact intermolecular interactions exist among the adjacent complexes. Fitting to the variable-temperature magnetic susceptibility data gave the magnetic coupling constant, 2J = –0.98 cm–1. Theoretical calculations, based on density functional theory (DFT) coupling with the broken-symmetry approach (BS), revealed that the π–π stacking magnetic coupling pathways resulted in weak ferromagnetic interactions with 2J = 4.86 cm–1 and 2J = 4.16 cm–1, respectively, for the adjacent NiII ions with separations of 8.568(19) Å and 8.749(32) Å, respectively; whereas the magnetic coupling pathway of the C–H···O short contact intermolecular interaction led to a weak antiferromagnetic interaction with 2J = –17.62 cm–1 for the adjacent NiII ions with a separation of 10.291(26) Å. The ferromagnetic coupling sign can be explained by the McConnell I spin-polarization mechanism.