Cobalt difluoride, structure

The anhydrous halides CoX2 may be made from the hydrated halides by heating or treatment with SOCl2 Cobalt difluoride is obtained by reacting CoCl2 with HF it has the rutile structure. Like the dichlorides of Mg, Mn, and Fe, blue CoCl2 adopts the CdCl2 structure. 

When subjected to high temperatures and pressures, polymorphs have been prepared for some of the difluorides. These all have distorted variants of the fluorite structure, with cubic, or pseudocubic, tetragonal cells. Manganese difluoride has been most studied (28-30) and similar polymorphism reported for cobalt and zinc difluorides (30). Recently, palladium and silver difluorides have been shown to behave in a similar way forming cubic metastable phases (31). In all cases there is a decrease in volume for the structure change. 

In contrast with the difluorides, the distribution of trifluorides extends to the third series of the transition metals, where iridium and gold trifluorides are fully characterized. In the second series, trifluorides are known for the elements from niobium to rhodium, with the exception of technetium, and in the first series, from titanium to cobalt. All the trifluorides have been characterized structurally, with earlier reports based on X-ray powder-diffraction data, since the compounds were not prepared in single-crystal form until more recently, when high-temperature, crystal-growth techniques became available. 

A comparison of the volumes per formula unit for the trifluorides is more complicated than for the difluorides because of the changes in structure type. If these changes are ignored, then some correlation with electronic configuration is possible. For the first transition series the volume decreases from scandium to chromium, increases from chromium to iron, and decreases to cobalt. This is as expected for the characteristic ligand-field stabilization effects (49), comparable with those for the difluorides. In the second series, there is a decrease from... 

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