The temperature dependence of the paramagnetic shifts in square planar ferrous complexes has been investigated for a number of porphyrin derivatives. Large anomalies in the Curie law have been observed in newly synthesized substituted tetraphenylporphins in which the environment of both faces of the tetrapyrrolic ring is strictly controlled. The pseudocontact and contact contributions to the measured hyperfine shifts as well as their temperature dependence have been interpreted in terms of a model with two electronic states. Following this model, the nature of the ground state of the ferrous ion depends critically upon the energy of the dz2 orbital relative to that of the degenerate dxz and dyz orbitals. For most of the porphyrin derivatives, the ground state is 3A2g, strongly perturbed by the closely lying 3Eg excited state. This strong mixing by spin–orbit coupling explains the large orbital contribution to the magnetic susceptibility of these complexes. A small axial perturbation can induce a reversal of the ground state with corresponding inversion of the magnetic anisotropy. Such a behavior, predicted by the model, is observed at low temperature by proton NMR spectroscopy of some complexes in solution.
No takes yet. Share an insight, caveat, or question.
Mispelter et al. (1980) studied this question.