A krypton difluoride coordination compound — where it acts as a ligand to a bromine atom — has been synthesized and studied.
Krypton, as expected for a relatively inert group 18 element, has a limited chemistry. Routes to all of its known compounds involve KrF2, which has been well characterized. There are a number of compounds featuring KrF+ and Kr2F3+, but the Lewis acid–base adducts of KrF2 (such as MoOF4·KrF2) have not been structurally characterized in full. Furthermore, there are no known compounds in which it acts as a ligand towards a main-group atom.
Now, Gary Schrobilgen and colleagues1 from McMaster University in Hamilton have prepared and fully characterized a KrF2 coordination compound, [BrOF2][AsF6]·2KrF2. They synthesized it by removing the XeF2 species from a similar compound under dynamic vacuum at −78 °C. Adding KrF2 to a suspension of [BrOF2][AsF6] in anhydrous HF produced the unusual compound. It was stable in anhydrous HF for several days at −78 °C and for over an hour at 25 °C.
X-ray diffraction measurements performed at −173 °C showed that the structure featured a central trigonal pyramidal BrOF2+ cation given a pseudo-octahedral geometry by the fluorine atoms from the KrF2 and AsF6 groups. The Kr–F bonds with bridging fluorine atoms are elongated by about 0.05 Å with respect to free KrF2. This indicates more covalent character than in KrF+ or Kr2F3+ salts, showing that in this case KrF2 is behaving as a coordinating ligand instead of a fluoride ion donor. The differences in Kr–F stretching modes shown by Raman spectroscopy confirmed this result, which was further explored computationally.
Change history
09 March 2010
In the version of this Research Highlight originally published, in the first paragraph the example of a Lewis-based adduct and final sentence were misleading, these have now been corrected.
References
Brock, D. S., Casalis de Pury, J. J., Mercier, H. P. A., Schrobilgen, G. J. & Silvi, B. A rare example of a krypton difluoride coordination compound: [BrOF2][AsF6] 2KrF2 . J. Am. Chem. Soc. 10.1021/ja9098559 (2010).
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Withers, N. The krypton factor. Nature Chem (2010). https://doi.org/10.1038/nchem.616
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DOI: https://doi.org/10.1038/nchem.616