The electronic and magnetic properties of ferrous iron in the substituted iron phthalocyanine, FePcOAr, exhibiting a true square-planar molecular environment, are investigated. Inhibition of intermolecular interactions by steric substituents, allows detailed investigation of the electronic structure arising from the planar geometry of the d6 electron configuration. Complementary magnetometry, Mössbauer, FD-FT THz-EPR and paramagnetic NMR spectroscopies show that FePcOAr has an S = 1 ground state with large positive, axial zero-field splitting and a strongly anisotropic g-tensor, with two g-values much larger than the free electron g-value and one smaller. Correlation between the magnetic properties and the electronic structure is provided by high-level quantum chemical calculations. The calculations indicate a nearly triply degenerate ground level, in which spin-orbit coupling mixes the isolated 3A2g ground state with two excited 3Eg states, whose energy gaps to the ground state are almost identical. These findings provide valuable insights in the electronic structure of phthalocyanines and the long standing discussion on their true electronic ground level, which has important implications for the application of this important class of complexes in catalysis and magnetic materials.