Chromium Jahn-Teller distortion

The series of 3d elements from scandium to iron as well as nickel preferably form octahedral complexes in the oxidation states I, II, III, and IV. Octahedra and tetrahe-dra are known for cobalt, and tetrahedra for zinc and copper . Copper(II) (d9) forms Jahn-Teller distorted octahedra and tetrahedra. With higher oxidation states (= smaller ionic radii) and larger ligands the tendency to form tetrahedra increases. For vanadium(V), chromium(VI) and manganese(VII) almost only tetrahedral coordination is known (VF5 is an exception). Nickel(II) low-spin complexes (d8) can be either octahedral or square. 

The same structure with a Jahn-Teller distortion even more pronounced was observed in RbCuFs 8). It is therefore beyond doubt, that the remaining compounds of Cu + and Cr2+ crystallize in the KCuFg-structure type, even if x-ray powder diagrams do not prove this. The lattice constants of the fluorides belonging to this group — some of the chromium compounds have been prepared only recently by Vollmer 316) — are summarized in the following Table 26. 

There is agreement over the gross structural features of these linear chain chromium(II) complexes (and the isomorphous copper(II) analogues), but different models have been used for structural refinement. Phase transitions associated with the Jahn-Teller distortion (or to fi to y as the temperature is lowered) complicate the structural studies. 

Transition metal difluorides are known mainly for first transition series elements, with palladium and silver difluorides from the second series, and no examples from the third. All these compounds have either the rutile structure, or, for chromium, copper, and silver, a distorted variant, which can be correlated with a Jahn-Teller distortion of the octahedral coordination of the ions. This rutile structure type is associated with smaller cations and, for comparison, although zinc difluoride has the same rutile structure, cadmium and mercury difluorides have the cubic fluorite structure with eight coordination of the cations (12). 

In the case of Cu2+ and Cr2+, compounds of which are susceptible to Jahn-Teller distortions ( 6.3 table 6.1), these cations are predicted to show strong preferences for the most distorted orthopyroxene M2 and amphibole M4 sites. A similar explanation accounts for the observed enrichments of Mn3+ ions in the distorted andalusite Ml, alkali amphibole M2, epidote M3 and, perhaps, epidote Ml sites (table 6.1). The presence of significant amounts of chromium in olivines from the Moon and as inclusions in diamond may be due to the presence of Cr2+ ions, and not Cr3+, in the distorted Ml and M2 sites of the olivine structure (Bums, 1975b), in which Jahn-Teller stability may be attained. A similar factor accounts for the stability and site occupancy of the Cr2+ ion in the orthopyroxene M2 site (table 6.1). 

Anhydrous chromium dihalides are conveniently prepared by reduction of the trihalides with H2 at 300-5()0 C, or by the action of HX (or h for the diiodide) on the metal at temperatures of the order of 1000°C. They are all deliquescent and the hydrates can be obtained by reduction of the trihalides using pure chromium metal and aqueous HX. All have distorted octahedral structures as anticipated for a metal ion with the d configuration which is particularly susceptible to Jahn-Teller distortion. This is typified by Crp2, which adopts a distorted rutile structure in which... 

The complex fluoride CrZrFe (Table 31) is an example of a class of compound A M Fs which undergoes phase transitions as the temperature changes. In the chromium(II) and copper(II) compounds additional modifications occur because of local and cooperative Jahn-Teller distortions. From the three spin-allowed d-d transitions for high-spin Cr in strongly distorted CrFe octahedra (Table 31) an octahedral A value of 7600 cm is calculated. Transitions between the monoclinic IT, pseudotetragonal I and cubic I phases have been... 

We illustrate these principles with three examples. In a d system, such as chromium(III), the ffee-ion ground state is " F, and in an octahedral field the electrons occupy the three lower (t2g) orbitals, giving a A2 ground state (Figure 5.11). There is no Jahn-Teller distortion in this case, but if there is tetragonal distortion, as in a complex... 

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