The electronic structures of [Fe(Por)(Im)O](1+) and [Fe(Por)(Im)O] (model compounds I and LI, respectively) have been studied on the basis of density functional theory or DFT (Por = porphine, Im = imidazole). The a(2u) pi-cation radical state ((4)A(2u)) was determined to be the ground state of compound I with total spin equal to 3/2, while the a(1u) pi-cation state ((4)A(1u)) was found to be 0.15 eV higher in energy than the (4)A(2u) state. Since, in both states, the spins were localized to the porphyrin ring (S = 1/2) and the Fe-O center (S = 1), the magnetic coupling interaction between the two spin sites was examined by using a broken symmetry method. The calculated J value revealed very weak magnetic coupling for the A(2u) state, which corresponded to the experimental data, The calculated J value revealed strong antiferromagnetic coupling for the A(1u) state. The calculated Mossbauer spectrum parameters (quadrupole splitting and asymmetry) were similar for both the A(1u) and A(2u) states, and both agreed well with experimental values. On the other hand, the calculated hyperfine coupling constants for the nitrogen and the proton of the porphyrin ring were different in the two states. Although the experimental coupling constant values of the pyrrole nitrogen atoms were intermediate between the calculated values for the A(2u) and A(1u) states, the experimental values for the meso protons were closer to the values calculated for the Ar, state, These results suggest that the electronic structure of compound I is closer to the A(2u) state than to the A(1u) state. However, these results also suggest that there is the possibility that the electronic structure of compound I is an admixture of the A(1u) state and the A(2u) state. The electronic structure of compound II was calculated and compared with the electronic structure of compound I. The energetics of the redox reaction between the two compounds is discussed.