Theory of cooperative high-spin⇄low-spin transitions in iron (III) compounds induced by the molecular distortions

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Abstract
The high‐spin?low‐spin transitions in iron (III) compounds are studied on the basis of the ligand field theory using the model in which the coupling between the iron ion and the intramolecular distortion is the driving force for the spin state transitions. The intermolecular coupling between the molecular distortions is taken into account and the distortions are treated dynamically. Two kinds of the molecular distortions, the uniform dilation and the uniaxial distortion, are considered. The discontinuous high‐spin?low‐spin transitions may be induced by the coupling with the uniaxial distortion, while the continuous spin state transitions arise from the coupling with the uniform dilation. Temperature dependences of equilibrium cooperative displacement of the ligands, high‐spin and low‐spin fractions of iron (III) ions, effective magnetic moment, quadrupole splitting in Mössbauer spectra, and internal magnetic field in NMR spectra are calculated based on the model as functions of the coupling strength between iron and the molecular distortion. It is shown that the essential features of the observed temperature dependences may be reproduced consistently in the various quantities by the present model.

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