with lone-pairs and their ionization potentials
- are often no greater than those of ammonia,
water, ethylene and other well-known ligands.
However, recent detailed studies of the interaction
of xenon and krypton with powerful
electron-pair acceptors, such as boron trifluoride
and trichloride, indicated that there
is no specific donor- acceptor bond-formation
even at very low temperatures.
This was the position at the beginning of
1962. Then, in ,June of that year, N. Bartlett
announced2 t hat when xenon was placed in
contact with gaseous platinum hexafluoride
at room temperature an immediate reaction
occurr ed to give an orange-yellow solid:
Xe (g) + PtF6 (g) = XePtF6 (s)
Xenon hexafluoroplatinate(v) was the first
authentic compound of a noble gas. The
idea was taken up by dozens of different
laboratories throughout the world, and within
18 months a 400-page book on the subject
had been published.
3 This gives some idea.
of the speed at which modern advances in
chemistry are made.
The first group to start 'vork after t he
initial discovery was one at the Argonne
National Laboratory in Chicago, and three
months after Bartlett's paper they reported4
that xenon combined directly with fluorine
when the two gases were heated in a nickel
vessel under pressure at 400° :
Xe (g) + 2F2 (g) = X eF 4 (s)
The compound can even be made simply by
passing a mixt ure of xenon and fluorine
diluted with nitrogen through a nickel tube
heated with a bunsen burner; white cryst als
of xenon t etrafluoride sublime out--an experiment
that could have been done at any
time during the past 60 years!
TABLE II
PROPERTIES OF THE XEKON FLUORIDES
Proper ty X eF 2 XeF4 X eF6
--- - ------ :W.p. (00) 140 114 46
Density (g/ ml) 4·32 4·04
Sol. in liq. HF at 25° (g/ 100 g) 175 4 250
H eat of solu tion (kcaljmole) 2·5 6·7 18
Vap. press. (mm H g a t 25°) . . 3 4 30
H eat of sublimation (k cal/
mole ) . . . . 12·3 15·3 9·0
Bond length Xe- F (A) 2·00 1·95 1·91
About 20 compounds of xenon have now
been isolated and many more identified in
r eaction systems. Compounds of krypton
and radon have also been made, and a
systematic account of this new area of
chemistry will now be given. Detailed
references to the original literature are given
in refs 1, 3, 5 and 6. It will be convenient
to break up the discussion into three parts:
(1) Preparation, physical properties and
structures ;
(2) Chemical reactions;
(3) Stability and bonding theory.
PREPARATION, PHYSICAL PROPERTIES
AND STRUCTURES
Xenon Difluoride
XeF2 is a white crystalline compound which
is most simply prepared by irradiat ing a
mixture of xenon and excess fluorine for one
day at 25° and 1! atm pressure with light
from a high-pressure mercury arc. It is important
to freeze out the product at - 78° as
it forms, otherwise some xenon tetrafluoride
is also formed, and these two compounds are
very difficult to separate. Amounts up to
10 g can be prepared in this way in silica
vessels or in nickel vessels with sapphire
windows. The compound can also be prepared
by irradiating a xenon-fluorine mixture
with 6
°Co y-rays, or by circulating a 1:4 mixture
of the gases through a hea ted nickel tube
at 400° and trapping the product at - 50°.
Some properties of xenon difluoride are summarized
in Table II. It can be seen that the
melting point, 140°, is far above the m.p. of
both xenon ( - 111 °) and fluorine (- 223°) and
the density is also greater than that of solid
xenon (3·1) or fluorine (1·3). Xenon difluoride
has a very high solubility in anhydrous
hydrogen fluoride, in which it is
non-conducting.
The vapour pressure is sufficient to allow
the compound to sublime readily when
warmed under vacuum and it forms beautiful
crystals in this way. The heat of vaporization
(12·3 kcaljmole) suggests that there is
appreciable separation of charge within the
molecule, and this coulombic attraction contributes
to the low volatilit.y ( cf. 6.H vap Xe