A theoretical comparative study of complexes of porphyrin (P), porphyrazine (Pz), phthalocyanine (Pc), porphycene (Pn), dibenzoporphycene (DBPn), and hermporphyrazine (HPz) with iron (Fe) has been carried out using a density functional theory (DFT) method. The difference in the core size and shape of the macrocycle has a substantial effect on the electronic structure and properties of the overall system. The ground states of FeP and FePc were identified to be the (3)A(2g) [(d(xy))(2)(d(z)(2))(2)(d pi)(2)] state, followed by E-3(g) [(d(xy))(2) (d(z)(2))(1)(d(pi))(3)]. For FePz, however, the E-3(g) - (3)A(2g) energy gap of 0.02 eV may be too small to distinguish between the ground and excited states. When the symmetry of the macrocycle is reduced from D-4h to D-2h, the degeneracy of the d(pi) (d(xz), d(yz)) orbitals is removed, and the ground state becomes B-3(2g) [(d(xy))(2) (d(Z)(2))(1)(d(yz))(2)(d(xz))(1)] or B-3(3g)[(...)(d(yz))(1)(d(xz))(2)] for FePn, FeDBPn, and FeHPz. The calculations also show how the change of the macrocycle can influence the axial ligand coordination of pyridine (Py) and CO to the Fe-II complexes. Finally, the electronic structures of the mono- and dipositive and -negative ions for all the unligated and ligated iron macrocycles were elucidated, which is important for understanding the redox properties of these compounds. The differences in the observed electrochemical (oxidation and reduction) properties between metal porphycenes (MPn) and metal porphyrins (MP) can be accounted for by the calculated results (orbital energy level diagrams, ionization potentials, and electron affinities).