Ligand field quenching

Ligand Field Quenching of Orbital Angular Momentum... 

However, useful as it is, ligand field theory is not a predictive first principles theory. Thus, it cannot be used to predict a priori the Mossbauer parameters of a given compound. Yet, the need to do so arises fi equently in Mossbauer spectroscopy. For example, if a reaction intermediate or some other unstable chemical species has been characterized by freeze quench Mossbauer spectroscopy and its SH parameters become available, then the question arises as to the structure of the unstable species. Mossbauer spectroscopy in itself does not provide enough information to answer this question in a deductive way. However, the more modest question which structures are compatible with the observed Mossbauer parameters can be answered if one is able to reliably predict Mossbauer parameters... 

There is an odd electron partially distributed between the dyz and dzx orbitals, which still retain their degeneracy and their rotational properties with respect to the 2-axis. The orbital angular momentum is thus not quenched by the ligand-field, and values of the magnetic moment as high as 4.1 B.M. are possible. The moment should be temperature dependent, and at low temperatures should drop rapidly towards zero, as the lowest state due to spin-orbit coupling is in fact nonmagnetic (74, 75). 

For the lanthanoid elements, ligand field splittings are so small that quenching of orbital angular momentum is not important. This probability also applies in the actinoid elements. 

In a very important study of the quartet-doublet reactivity question,72 it was found that ligand-field irradiation of [Cr(CN)6]3 in degassed dimethylformamide results in both phosphorescence from the doublet and substitution of cyanide. In air-saturated solution, however, phosphorescence is quenched completely while the photoreaction is unaffected. The two processes appear to be uncoupled and thus originate from different excited states. The most straightforward conclusion is that the lowest quartet excited state in [Cr(CN)6]3 is the sole precursor to photoreaction. 

It is important to mention that a reduction of ligand field symmetry can strongly alter the magnetic properties of ions If the symmetry of a 3d1 system with an Oh-symmetric ligand field is reduced (e.g., by an orthorhombic distortion), the entire orbital moment will be quenched and spin-only magnetism is observed. 

Comparison of net spins of cations in strong ligand fields with those for the free cation illustrates spin quenching.)... 

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