Mössbauer studies of bonding in iron porphyrin-ligand systems

Mössbauer and electron paramagnetic resonance (EPR) spectra of several iron porphyrins have been obtained. The Mössbauer data for a series of high-spin pentacoordinated iron (III) compounds establish that both the electric field gradient and the zero-field splitting of the ionic S22 states follow the order fluoro< acetato<azido<chloro< bromo. The correlation of the two parameters can be qualitatively rationalized by considering the strong perturbing influence of the porphyrin ring on the relatively large high-spin ion. The implications of the data for recent detailed theoretical models of the iron porphyrins are discussed. In pyridine, chloroprotohemin displays a Mössbauer spectrum with unusual temperature dependence and quadrupole splitting like that observed for acid methemoglobin. This suggests that these features may arise because of stronger interactions between orbitals of the more nearly planar metal ion with the porphyrin ring in hexacoordinated iron porphyrins. The isomer shifts of the Fe (II) dipyridine porphyrins are nearly the same as those of the Fe (III) forms, indicating that very extensive electron delocalization from the iron takes place in the former case. The formal valence change does not, then, result in a large difference in charge localized on the iron.