How does xenon bind

Fascination with chemistry

Fig.1. shows the anion XeF5- in the connection (CH3)4N+ XeF5-, a derivative of the XeF4 , has the form of a regular pentagon, which is rare in chemistry. (Konrad Seppelt, FU Berlin, based on [2])

Almost all of these connections are structurally easy to describe, they obey the rules of the VSEPR model: e.g. are XeF2 and its derivatives structured linearly, XeF4 and its derivatives are square, XeO4 on the other hand constructed tetrahedral. A famous exception is the structure of the XeF6. According to the above model, this should not be octahedral. After dozens of studies, both experimental and theoretical, it is best described as a slightly dynamically distorted octahedral molecule. In the condensed phase it mainly associates to a tetramer, trimers and octamers have also been detected.

The search for xenon compounds in other binding states was not very successful for a long time, but there were indications early on: With a reduction of XeF2 in SbF5/ HF solution in which it is called XeF+ is present, a green solution is created, in which correctly the Xe-Xe+ Cation was suspected, as it was decades later by the crystal structure analysis of the Xe2+ Sb4F.21- could be proven with a very long bond (308 pm) between the xenon atoms. Under higher xenon pressure at room temperature, even a blue, linear Xe is formed in solution4+ Cation. However, in this case the Xe-Xe bonds are to be assumed to be even weaker [3].

The idea of ​​binding xenon as a ligand in transition metal complexes has also been under discussion for a long time. There were already spectroscopic indications. But it was a surprise when xenon complexes could be isolated in mono-, di- and trivalent gold compounds with astonishing stability, up to and including AuXe42+. [4] Also a HgXe2+ Ion could be isolated. These few xenon complexes have remained the only ones. The question of why this is so has not yet been answered. According to qualitative considerations and calculations, there should actually be more if the metal atoms are cationic, and because of the relativistic effect that apparently strengthens the bond between xenon and metal atom, it is primarily heavy transition metals as binding partners.