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THE COMPOUNDS OF THE; NOBLE GASES
By G. J. MOODY,B.SC., PH.D. and J. D. R. THOMAS,M.SC., F.R.I.C.
Welsh College of Advanced Technology, Cardi$
and an excess of fluorine through a nickel tube at 400°C
and trapping out the product at -50°C.
In this way,formation of xenon(1v) fluoride by the stage

In August of this year, seventy years will have elapsed
since the discovery and isolation of the first noble gas,
namely argon, by Lord Rayleigh and William Ramsay;lbut barely two years have passed since the preparationof the first compound of a noble gas.
Even though claims of compound formation by thenoble gases have been made since their discovery, thefirst authentic compound was xenon hexafluoroplatinate(v), Xe+PtF,-, reported by Bartlett in June, 1962,,
as the result of an idea arising from unrelated researcheson the synthesis of dioxygenyl hexafluoroplatinate (v),0,+PtF6-.3*4 It was the similarity of the first ionization
potential of molecular oxygen and of atomic xenon atjust over 12 eV, that led to Bartlett's forecast.
By today, several of these xenon metal fluorides areknown, as well as a succession of other xenon compounds,including fluorides, an oxide and oxyfluorides. Inaddition, two fluorides of krypton are known and alsoa radon fluoride.
The remarkable feature about the xenon metal
fluorides is that they are formed at room temperatureby simple reaction of xenon with the appropriate
metal(v1) fluoride, MF,, according to the equation+F2 --f XeF, . .
- * (2)XeF,
can be checked.ll Any xenon(1v) fluoride present as
impurity can be separated by flushing the vapours fromthe system.14
Ultra-violet photolysis of xenon and fluorine in an
all-nickel vessel fitted with sapphire irradiation windows
gives xenon(11) fluoride with only a trace of xenon(1v)fluoride.le In addition, ultra-violet irradiation at
20°K of fluorine, xenon and argon mixtures deposited
on caesium iodide crystals yields xenon(I1) flu0ride.l'This method, known as the matrix-isolation technique,
has also yielded krypton(I1) fluoride from mixtures offluorine, krypton and arg0n.l Krypton(11) fluoride
has also been obtained by bombarding krypton and amodest excess of fluorine with 1.5 meV electrons at- 150"C.l8
Returning to xenon(11) fluoride, this compound has
also been obtained as condensate on a cold finger at-78"C, by passing discharges provided by an induction
coil through mixtures of xenon and f l u ~ r i n e . ~ ~ , ~ ~XeMF, -+ Xe+MF6-.
(1)
However, an important development in the preparationof xenon(11) fluoride is the dispensation with elemental
In this way, compounds have been prepared where
M = Pt,, P U , Ru5~and Rh.6 However, the reaction
fluorine in the preparation by high-voltage discharge ofa mixture of xenon and carbon tetrafluoride. 21 Successdoes not proceed with uranium(vI), neptunium(v1) andful preparations have been similarly obtained withiridium(v1) fluoride^.^Pyrolysis of xenon hexafluoroplatinate (v) gives xenon
CF,OF and FSO,F.,,The only other simple fluoride of the noble gases thatand a red solid of composition, Xe(PtF6),.6
has been obtained to date is xenon(v1) fluoride prepared
Direct reaction between xenon and fluorine at atemperature of 400°C gives rise to xenon(1v) f l ~ o r i d e , ~ by heating xenon and excess of fluorine to temperatures
above 300"C,very high pressures being also
the product being obtained by cooling to -78°C and~sed.23~24
3pumping off the excess fluorine. Similar conditions
The three xenon fluorides are colourless solids,
have not resulted in the formation of the corresponding
the room-temperature vapour pressure of thekrypton compound, although radon(1v) fluoride hasxenon(11) and xenon(1v) compounds being aboutprobably been obtained in this way.9 In this last case,assessment of the product has not so far been possible
3 mmHg, while that of the xenon(v1) compound at
20°C is 27 mmHg.5,7~11s20s23,25
Melting points range
because of the small yields, due to the minute amountfrom 46°C for xenon(v1) fluoride,,, through 100°C forof radon available and the inherent radiation hazard
xenon(1v) fluoride7 to between 120"C20 and 14O"Cl1
arising from the intense radioactivity.
for xenon (11) fluoride.
High-temperature flow systems have also been used
Crystallographic studies have shownZ7-3O that xenon
to prepare xenon(1v) fluoride.l0~l1,l2Both xenon(1v) fluoride13 and krypton(1v) fluorides(IV) fluoride crystals are of the monoclinic system ofdensity 4-04 to 4-10 g ~ m with
- ~ a second, less stable
are obtained by passage of high-voltage discharges
rnodifi~ation~~
having density 4.42 g ~ m - ~ The.
xenon
through mixtures of the appropriate gaseous elements(11) fluoride crystal28is body-centred tetragonal of density
at very low temperatures.
4-32 g ~ m - ~ .
Xenon(I1) fluoride has been reported, and identified57l4915as an impurity in xenon(1v) fluoride preparaThe structures of the xenon fluorides have been
tions. The compound is obtained by circulating xenon
compared with the polyhalides, and, by analogy with+239

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JOURNAL OF THE ROYALINSTITUTE
OF CHEMISTRY

32[FEBRUARY

halogen fluorides and chlorides, a linear structure
thermal datum available in the literature for the krypton
based on the trigonal bipyramid sp3d hybridized arrangefluorides is the value of 8.84 kcal mole-l for the enthalpy
ment of electron pairs is proposed for xenon(r1)of sublimation of the krypton(1v) compound.8
f l u ~ r i d e , ~ while~ , ~ ~a, ~square~planar arrangement,
Product stoichiometry has, in many cases, been
based on the octahedral sp3d2 arrangement of electron
assessed by reactant consumption ratios, but krypton ( ~ v )pairs, is suggested for the xenon(1v) ~ 0 m p 0 ~ n d . ~ ~ 9fluoride~ ~ 9 ~is~sufficiently unstable for volumetric assay ofHowever, the octahedral geometry of xenon(v1) fluoride
the recovered krypton and fluorine to provide a further
cannot be accommodated on models such as these, andcheck on the formula.8 For the relatively more stable
an alternative view in which the d orbitals of the noble
xenon fluorides, product stoichiometry has been congas are of little importance, has been promoted.
firmed by reduction with hydrogen,This proposal, where the bonding has been ascribed toXeF,n H 2 -+ XenHF. . (3)delocalized molecular orbitals formed mainly by a2
three-centre four-electron combination of p a-type
followed by analysis of the products.
Similarly,
xenon and fluorine orbital~,31~35~36~~7~38
predicts theconfirmationcanalso
beobtainedbyreductionwithcorrect geometries for all three xenon fluorides. The
26
The
mercury,givingquantitativexenon
re1ease.l3*
remote possibility of a tetrahedral xenon(1v) fluoride
formulaearealsopartially
verifiedontheevidenceof
has not been completely discounted.
ion
species
obtainedby
mass~pectrometry.69~~925
The linear and square planar configurations for
Xenon ( ~ v )fluoride dissolves in liquid antimony(v)xenon(11) fluoride and xenon(1v) fluoride, respectively,fluoride
and in tantalum(v) fluoride to give the respective
have been confirmed by spectroscopic
additioncomplexes, XeF,,2SbF5 and XeF,,2TaF5.52
including n.m.r. ~ p e ~ t r 0 ~ ~ 0 p yas, ~well~ s ~as~ by~~~9~~The
iodideion in aqueous solution liberates xenon,
X-ray5J09 28 and ne~tron-diffraction2~~~7
investigations.
iodineandthefluoride
ion from xenon fluorides.12920921926
Infra-red spectra have established a symmetrical
Thereaction
issometimes
complicated by simultaneous
structure for xenon(v1) fluoride,23 but on the basis of anhydrolysis.
infra-red absorption band at 1225 cm-l, Agron et aZ.ll
Of the other reactions of the xenon fluorides reported
favour covalent bonding based on a xenon valence shellinthe literature, the most notable are the hydrolyses,of sp3d3 orbitals instead of the octahedral structure.
sincethese give rise to a range of other xenon compounds.
On the evidence of X-ray crystal-structure investigaEven though the hydrolysis of xenon(1v) fluoride cant i o n , l O ~and~ ~ of the more precise method of neutron
be represented by the equationd i f f r a c t i ~ nthe
, ~ ~Xe-F bond length in xenon(1v) fluoride
2H,O -+ Xe
4HF
0, . . (4)XeF,
is 1-95A. The value of 1-85A obtained for the gas
molecule from vibrational spectroscopic studies4, is infor a wide pH range, the reaction is certainly morereasonable agreement. Within the limit of expericomplex,5~and xenon(v1) oxide has been preparedmental uncertainty, the molecule of xenon(rv) fluoride
by hydrolysis of xenon(1v) fluoride,53~~~
as has also thespecies designated, Xe(OH)4.55
is regarded as square planar with four equal F-Xe-F
Pyrolysis of a mixture of xenon and fluorine containing
angles of 90°.10927929
a fractional percentage of oxygen first provided evidence
The accepted Xe-F bond length in the linear xenon(11)fluoride is the neutron diffraction value of 2.00A.47
for the existence of the oxyfluorides, XeOF, and/orXeOF4.6s15 Other oxyfluorides for which tentative
The value obtained for the gaseous molecule (1-7A)
experimental evidence is available include Xe02F,,
from the infra-red vibrational P-R spacing is much
lower.l1
XeO,F,,33 XeOF,33 and Xe0,F,.33
The bond energies of the Xe-F bonds in xenon(I1)
XeOF,, a colourless liquid, has also been obtained byfluoride, 22948949 as well as in xenon(1v)
the incomplete hydrolysis of xenon(v1) fluoride ; comand xenon(v1) fluoride,22 fall around 30 kcal per
24
plete hydrolysis gives xenon ( v ~oxide.
)
The structure of the xenon oxyfluorides has been
Xe-F bond. These have been calculated from enthalpies of formation deduced t h e o r e t i ~ a l l yand~ ~ by~ ~ ~ explained on the basis of the oxygen atom completing
e~periment.~~950
its octet by attachment to a lone pair of electrons on theEnthalpies of sublimation are availcentral xenon atom.33 In the case of the odd-electron
able for these fluorides51 as well as for the xenon(v1)
XeOF,, it is believed that the oxygen atom may be afluoride.
The krypton fluorides are less stable than the corresdirect substitution for a fluorine, being satisfied by thepartial sharing of a single electron.39 Infra-red andponding xenon compounds.8J8 While no data appear
to have been published for krypton(1v) fluoride, theRaman spectra are in agreement with the square pyramid
arrangement for XeOF,. 24
krypton(I1) compound has been assigned a linear
Xenon(v1) oxide was first detected by accident in thesymmetry on infra-red evidence.17 The parity of thecondensate of xenon(v1) fluoride.5s The compound isforce constants for this, and the xenon(I1) compound,indicates that the bond energies for the two compounds
also obtained by the hydrolysis of xenon(v1) fluoride,cannot be far removed.l7 The only experimental
and of XeOF4.24966
22931734++7916925

7939

2192337911940941942

279

26950

+++

22933

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THECOMPOUNDSOF THE NOBLE
GASES

19641

With the possible exception of krypton(I1) fluoride,lsxenon(v1) oxide is the most unstable compound of thenoble gases so far examined.55 It is particularly
explosive and in this respect simulates T.N.T.57 Its
stoichiometry has been confirmed by its reaction withacidified potassium iodide :54
XeO,
6Hf
61- -+31,
Xe
3H,O . . ( 5 )Crystallographic and infra-red spectroscopic data
confirm the molecule as having a trigonal bipyramid
geometry similar to the isoelectronic iodate ion,24754y56
the average Xe=O bond length being 1.76A and theaverage O=Xe=O bond angle, 103°.54
Xenic acid, Xe(OH),, is claimed to be the ionic xenon
species resulting from the hydrolysis of xenon(1v)fluoride5 and xenon(vr) fluoride.5s The compound,which can be stored at -30°C for several weeks,58
results from the slow hydrolysis of xenon(v1) fluoride, orxenon oxyfluorides, at O"C.58 Salts of the acid are wellestablished and can be prepared either by the additionof alkali to an aqueous solution of the acid,58 or byalkaline hydrolysis of xenon fluorides.59 In the latter
case, perxenates are obtained with the octahedral
Xe0,-4 ion containing XevlIr species.59The silver22andsodium59salts are known as well as the acid, H,Xe06.22
Acid or neutral hydrolysis of the xenon fluorides gives
rise to the xenon(v1) species, Xe06-6, and the barium,
sodium and potassium salts have been prepared.5s
It is clear that with the characterization of about
20 compounds of this family of elements, the title 'inert'
is no longer justified and the term 'noble' has, therefore,
been used throughout this brief survey. However,compounds are yet to be prepared for helium, neon andargon, but except for the possibility of ArF,, these are,at present, considered to be ~ n l i k e l y . ~ ~ * ~ *In less than two years (June, 1962, to December, 1963),
about 60 publications have appeared in this rapidly
expanding branch of inorganic chemistry, and while themajority of these have been short communications, thefield has been discussed at conference level,60 and ashort review on the subject has also been published.61++++REFERENCES

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