Theoretical model study of the quantum-mechanically mixed-spin (S=5/2,3/2) ground state in iron (III) porphyrins

After a selective review of some recent studies on structural and spin properties of iron–porphyrin complexes, the role of axial ligands and the porphyrin-structure effect on spin state is presented. It is shown that in crystal-field approximation the structure effects on spin-state differences in five- and six-coordinate complexes can be rationalized within the available x-ray data. In the framework of a vibronic-coupling analysis, added to the ‘‘charge–repulsion’’ model of Saito and Kashiwagi, the possibility of a drastic reduction in the energy needed for triggering the intermediate (S=3/2) to high-spin (S=5/2) transition is numerically demonstrated. In addition, two supplementary quantum-mechanical effects through axial ligands on bond distances and spin-state preferences are described and illustrated by model calculations. The results are found to agree with the experimental data.