Page 322 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE NOBLE GASES
302
+
• A Xe–F unit may also act as a source of electrophlic F . Accordingly, XeF is
2
sometimes used in organic synthesis as an electrophilic fluorinating agent.
• Krypton difluoride (KrF ) can be synthesized from the elements in a number of dif-
2
ferent ways, including low temperature photolysis. It is considerably less stable than
XeF , as shown by the following reaction enthalpies:
2
KrF (g)= Kr(g)+ F (g) ΔG =−63 kJ∕mol (8.1)
0
2 2
0
XeF (g)= Xe(g)+ F (g)ΔG = 105 kJ∕mol (8.2)
2
2
∘
KrF can be stored indefinitely at −78 C, but decomposes slowly at room tempera-
2
ture. It is an exceedingly powerful oxidant.
• Radon, on the other hand, reacts readily with F , yielding radon difluoride. The com-
2
pound has not been fully characterized. It decomposes on vaporization and is believed
to be an ionic compound. The strong radioactivity of all of radon’s isotopes has dis-
couraged detailed studies of its chemical properties; the half-life of 122 Rn, the longest
lived of all radon isotopes, is only 3.82 days.
Most of the discussion below will focus on xenon. We trust that you’ll find noble gas
chemistry, a term that a bit over 50 years ago would have been an oxymoron, fascinating.
Indeed, a few of the mechanisms should be as challenging as anything you have encountered
in the rest of this book.

REVIEW PROBLEM 8.1
The first ionization energies of Kr, Xe, and Rn are 1351, 1170, and 1037 kJ/mol,
respectively. Use this information to rationalize (i) the instability of KrF relative to
2
XeF and (ii) the likely ionic character of RnF .
2
2

8.1 THE XENON FLUORIDES: FLUORIDE DONORS AND ACCEPTORS

All three fluorides XeF (n = 2, 4, 6) are prepared by the action of elemental F on Xe under
n
2
various conditions of temperature and pressure. Almost certainly, the mechanisms involve
radical reactions of F atoms. We won’t be discussing them here, but feel free to sharpen
your skills with fishhook arrows by writing out some of them.
The compounds act as both fluoride donors and acceptors. Thus, with tetramethylammo-
nium fluoride in acetonitrile under rigorously anhydrous conditions, XeF forms the unique
4
−
pentagonal-planar (D )[XeF ] anion:
5h
5
F
F F F − F
Xe Xe
F F (8.3)
− F F
F