Page 336 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

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THE NOBLE GASES
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8.12 PLUS ULTRA

In this final vignette, we’ll skip arrow pushing and attempt to present a broader perspective
of noble gas chemistry. Not long ago, Seidel and Seppelt at the Free University of Berlin
∘
treated AuF in HF/SbF with xenon, obtaining a dark red solution at −40 C, which yielded
3
5
∘
crystals of [AuXe ](Sb F ) at −78 C(see Further Reading for key references). The
2 11 2
4
researchers’ intention was to obtain the simple but elusive compound AuF, using xenon
as a very mild reducing agent. Instead, they ended up with the astounding cationic species
2+
[AuXe ] , made up of two of the most unreactive elements in the periodic table—gold and
4
xenon. Subsequently, the same research group synthesized other Au–Xe complexes such
2+
as [AuXe ] . Simple molecular orbital arguments do not appear to provide a rationale for
2
the stability of these species. More advanced theoretical studies suggest that their stability
owes a great deal to relativistic effects, which are known to be important for gold.
This chemistry is a good reminder that arrow pushing “explains” a lot, particularly the
“how” of chemical reactions, but not the “why.” Other concepts such as chemical bonding
and thermodynamics therefore must not be ignored. The reaction is also an inspiration that
truly fundamental discoveries are waiting to be made. The old motto Plus ultra (further
beyond!) continues to be an appropriate one for inorganic chemistry.
8.13 SUMMARY

The following are some of the highlights of what we discussed in this chapter.

1. Of the three noble gases with a significant chemistry (Kr, Xe, and Rn), the chemistry
of Xe is by far the most developed, with the three molecular fluorides XeF ,XeF ,
2 4
and XeF serving as starting materials for most other Xe compounds.
6
2. The xenon fluorides in general act as both fluoride ion donors and acceptors.
3. Toward certain nucleophiles, xenon fluorides act as a source of electrophilic fluorine,
+
that is, F ion equivalents. Thus, commercially available xenon difluoride is a useful
+
F donor in organic chemistry.
4. The xenon fluorides react vigorously with water. While XeF hydrolyses straightfor-
6
wardly to XeO ,XeF and XeF decompose by more complex redox pathways.
3 2 4
5. Xenon–nitrogen and xenon–carbon bonds involving highly electronegative N or C
ligands have been synthesized. Among other strategies, organosilicon chemistry has
been creatively used for this purpose.
6. KrF , one of the strongest oxidants known, has been used to used to effect a num-
2
ber of unique transformations; perhaps the most notable of these is the formation of
Au(V)-fluoride compounds.