Distortions, from perfect octahedral symmetry

Figure 5.4 (A) Olivine structure dashed line contours elementary cell (axes c and b). (B) Details of coordination state and distortion of Ml-Ml sites 0(1), 0(2), 0(3) oxygens occupy nonequivalent positions distortion from perfect octahedral symmetry is more marked for Ml site.

So far, we have discussed the crystalline field acting on the ion A due to an octahedral environment of six B ligand ions. In many optically ion activated crystals, such as Ti +rAlaOj, the local symmetry of the active ion A is slightly distorted from the perfect octahedral symmetry Oh symmetry). This distortion can be considered as a perturbation of the main octahedral field. In general, this perturbation lifts the orbital degeneracy of the tag and eg levels and then produces additional structure in the tag eg absorption/emission bands. 

In the met form, the two ferric ions are bridged by an oxo ligand and the carboxylate side chain of a glutamate residue. The Fel atom has a distorted square pyramidal geometry, alternately characterized as pseudo-octahedral if the aspartate residue is assigned as chelating bi-dentate, whereas Fe2 has nearly perfect octahedral symmetry. In the active form of the enzyme, Tyr 122 has been oxidized to a tyrosyl radical (9), which, as revealed by the X-ray structure of met R2, is located in a hydrophobic pocket with its hydroxyl group 5.3 A away from Fel. The pocket does not contain any oxidizable side chains near the radical site... 

The conformation of lowest energy appears to be that of C3v symmetry. Part of the experimental difficulties stems from the fact that the molecule is highly dynamic and probably passes through several conformations. In either of the two models shown in Fig. 6.13, the Xe—F bonds near Ihe lone pair appear to be somewhat lengthened and distorted away from the lone pair however, the distortion is less titan would have been expected on the basis of the VSEPR model. That the latier model correctly predicted a distortion at all at a time when others were predicting a highly symmetrical octahedral molecule (all other hexafluoride such as SF6 and UF are perfectly octahedral) is a signal success, however. 

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