Density functional theory (DFT) was employed to study the impact of Mg2+ ions on the o-semiquinone radical anions of different aromaticity in protic and aprotic solvents. After the geometry Optimization of ligands and complexes, their g tensors were computed at the UBP86/TZVP and UB3LYP/TZVP theory levels. The suitability of various model systems, assuming continuum dielectric approaches, different Mg2+ coordination spheres (completed by solvent molecules), and inclusion of additional solvent molecules H-bonded to the ligands, was tested in terms of correlation between the experimental and calculated g-shifts. The effects of complexation, ligands aromaticity, and solvents on the electron spin density for o-semiquinones are discussed. To recognize clearly the changes in the nature of the g tensor components, the contributions from particular excited states were analyzed. A structural characterization of the tested complexes is expected to be helpful in investigations on the complicated biosystems in which the similar paramagnetic units are present.