Co- and Counterrotation of Magnetic Axes and Axial Ligands in Low-Spin Ferriheme Systems

Abstract

The orientation of the principal axes of the g tensor with respect to the relationship of axial ligand planes to the porphyrin nitrogens has been studied in the framework of the one-electron crystal field model for tetragonal and rhombic low-spin d5 complexes such as ferriheme centers. All five d atomic orbitals were taken into account for two different ground-state electronic configurations, the “normal” (dxy)2(dxz,dyz)3 and the “novel” (dxz,dyz)4(dxy)1 configurations. The expressions for the g tensor, g values, and magnetic axes were derived on the basis of first-order perturbation theory. The conditions for co- and counterrotation of magnetic axes with rotation of planar axial ligands away from the porphyrin nitrogens toward the meso positions and beyond, as well as the order of g values, have been analyzed. It is found that counterrotation is the only possibility for the (dxz,dyz)4(dxy)1 configuration and that it is also by far more common for the (dxy)2(dxz,dyz)3 electron configuration. The possibili...