Low symmetry ligand fields

Because they can, and usually do, remove orbital degeneracies—and thus reduce the orbital contribution to the magnetism—low-symmetry ligand fields cannot be ignored in a study of the magnetic properties of transition metal complexes. Unfortunately, it is no easy matter to determine the splitting effects of low symmetry fields usually it is necessary to work with two 

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Most tetravalent vanadium complexes contain the V02+ unit which leads to a strong low-symmetry ligand field component. Three ligand field bands with 20 commonly occur, one — 13000 cm-1, one 17000 and one 25000-30000 cm-1, in VOL4 units, but their classification in formal C4v. symmetry is not clear, in spite of the availability of polarized light results.38-138-139 However, the ground term is accepted to be 2B2. 

Finally, it has to be recalled that internal strains or crystal packing forces have to be present, in order to stabilize one of several equivalent minima so that the system is seen as statically distorted. It has been stated that even small low-symmetry perturbations can be amplified by vibronic coupling A mechanism of this kind may be more effective in lifting electronic degeneracies than low-symmetry ligand field components. The important point is whether JT contributions are greater or comparable with the low-symmetry ones if it is so, JT terms must be included in to the Hamiltonian and have to be considered before or together with the static low-symmetry crystal field. 

The complex ion CrOBrj" has been isolated as the relatively stable bipy" salt. The value of //, 1.75 BM, and the g value, 1.999, show, as expected, that the orbital moment is quenched in a low-symmetry ligand field, the distortion being caused by the strong metal-oxygen bond. 

Compounds with close to octahedral stereochemistry, e.g. TiXs, including some with a mixture of ligands, have /ie (300) 1.8 BM decreasing to 1.5 BM at 80 K in a manner conforming to the term with a low-symmetry ligand field component (Ar), although a unique determination of A, A and A is often not possible. Although others have formal octahedral stereochemistry [Ti(H20)6, the electronic structure seems to correspond to low symmetry /ieir(300) 1.7 BM, ieff(80) w 1.6 BM. 

An exact calculation of the orbital separations may be performed without resorting to perturbation theory. Let the combined Hamiltonian of the spin-orbit coupling and the low symmetry ligand field, V, be... 

The second effect which has been neglected is that due to low-symmetry crystal fields. As the discussion in this chapter has been confined to octahedral MLg complexes one might assume that low-symmetry ligand fields could safely be ignored, but this is not so. A theorem due to Jahn and Teller states that any non-linear ion or molecule which is in an orbitally degenerate term will distort to relieve this degeneracy. This means that all Ey, and T2g terms of d" configurations, in principle, are unstable with respect to some distortion which reduces the symmetry. Of course, as... 

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