The krypton factor

A krypton difluoride coordination compound — where it acts as a ligand to a bromine atom — has been synthesized and studied.

Credit: © 2010 ACS

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.