The Most Noble Complex Compound

Bartlett's oxidation of Xe using PtF₆ 

The noble gas compounds have been a great interest since the creation of its kind, XePtF₆, in 1962. Since then, many xenon compounds with direct bonds to fluorine, oxygen, nitrogen, carbon, xenon itself, and chlorine have been established. However, all these bonded atoms are electronegative main-group elements but there have been several indications that metal-xenon bonds can be formed and detected such as (CO)₅Mo...Xe. Besides that, The Au-Xe⁺ has been detected by mass spectroscopy and has been calculated with a best estimate for the bond distance of 257 pm. Researchers from Germany successfully created the first metal-xenon square planar compound with direct Au-Xe bond.

The original study of this synthesis was the preparation of the mysterious gold(I) fluoride, AuF, via reduction of AuF₃ in the presence of weakly coordinating agent. The initial experiment was using AsF₃ in HF/SbF₅ as reducing agent and it afforded the first derivative of AuF. Then, by replacing with milder reducing agent Xe, the reduction stopped at Au²⁺ and resulted the unexpected dark red crystal [AuXe₄]²⁺ that can be grown at -78 °C. Removal of Xe under vacuum results in the known Au(SbF6)₂. Then, addition of Xe to Au²⁺ in HF/SbF₅ gives a dark red solution at -40 °C. It is noteworthy that the reaction between Xe and Au²⁺ is reversible and it requires a xenon pressure of 10 bar to stabilise [AuXe₄]²⁺.

The crystal structure of the cation [AuXe₄]²⁺ 
in [AuXe₄][Sb₂F₁₁]₂.

The existence and structure of this novel compound was revealed from a single-crystal structure determination where it exists as [AuXe₄][Sb₂F₁₁]₂. The cation forms a regular square with Au-Xe bond lengths ranging from 273 to 275 pm, and three weak contacts between the cation and the anion complete the coordination sphere around gold atom. This compound is stable at -40 °C and warming it above this temperature results in liquidification, loss of gaseous Xe, and colour change from dark red to light orange.

Interestingly, in this complex Xe acts as σ donor towards Au²⁺ and this is reflected in the calculated charge distribution where the main part of the positive charge resides on the Xe atoms. This large charge transfer may be explain due to strong relativistic effect on gold atom. Hence, the bonding between xenon and gold in this compound may not be different from that between xenon and any electronegative main-group elements.

During the reduction and the complexation, the presence of extreme Brønsted acid HF/SbF₅ is essential. The Au³⁺ ion in fluoride systems is normanyl as [AuF₄]^-, which then protonated to form Au(HF)₄, which can be considered as naked Au³⁺; hence, it has a much higher oxidation potential.

[Au(HF)_n]²⁺ + 4Xe ⇌ [AuXe₄]²⁺ + nHF 

Then, the complexation reaction must be an equilibrium, which again can only proceed if Xe is the strongest base in the system.

AuF₃ + 6Xe + 3H⁺ → [AuXe₄]²⁺ + [Xe₂]⁺ + 3HF

Furthermore, in the overall reaction indicates again the role protons and the formation of green [Xe₂][Sb₄F₂₁] crystals, which was detected in the solid reaction mixture at -60 °C.

This study demonstrated a novel Xe compound where it is bonded to a metal but the isolation of [AuXe₄]²⁺ raises many questions such as whether this compound remains unique or if this is the first of a series of a new complexes. Besides that, it is difficult to predict the nature of this complex since it is unique in another way: stoichiometry and structure of Au²⁺ complex are rare if not entirely new.

Reference
S. Seidel and K. Seppelt, Science, 2000, 290, 117-118.