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Encyclopedia of Physical Science and Technology EN007D-343 July 10, 2001 20:13

830 Inorganic Exotic Molecules

least basic of the lot and binds a proton by almost 180 kJ of isolation as a solid salt. Xenon forms numerous species
−1
mol . While this so-called proton affinity is small, and with the oxidation state +2 starting with XeF 2 and with a
indeed helium is the least basic of all neutral species, this variety of other electronegative groups as well as salts con-
+
binding energy is comparable to the iodine–iodine bond in taining the fluorine-conjugate acids [XeF] and [Xe 2 F 3 ] +
elemental I 2 as well as the oxygen–oxygen bond in many ions.
+
peroxides. [HeH] is rare and exotic, not because of its There are also numerous species of the type HXeY that
+
−
σ bond and being isoelectronic to the well known H 2 ,but are perhaps best described as ion pairs HXe Y that are
because the helium–proton bond is both weak and labile: frozen in their matrix and so cannot revert and rearrange
proton transfer to anything else in its chemical environ- by proton transfer to form Xe + HY from whence they
ment is exothermic and rapid. Isoelectronic to [HeH] + came. The tetravalent and hexavalent XeF 4 and XeF 6 have
2+
is [LiH] , but this latter species is seemingly unknown chemistry generally not unlike XeF 2 . A big difference is
and unbound. Unbound, but not unknown, is the isoelec- their anion chemistry: the former reacts with fluoride salts
−
2+
tronic [He 2 ] . This seems like a contradiction in terms: to form the aforementioned [XeF 5 ] ion, the latter is the
what we mean to say is that there is a potential well, the thermally highly stable [XeF 8 ] 2− with its nine electron
species has a finite lifetime, but it is unstable relative to its pairs but the very rare square antiprism geometry. Fur-
separation products 2He . We will not discuss its metasta- thermore, the hydrolysis of XeF 4 and XeF 6 readily gives
+
bility other than to mention that it is related to the nono- the explosive, isolable (at one’s risk) xenon trioxide and
ligomerization of neutral helium and the slowness of most xenon tetraoxide. Interestingly, the equally sensible xenon
radioactive decay by α-particle emission. The process is monoxide and dioxide remain uncharacterized and all but
called “tunneling,” a wonderfully exotic phenomenon that unknown and unstudied, a situation reminiscent of the va-
is regrettably more in the province of the physicist than lence isoelectronic oxyanions of sulfur. There is much that
the inorganic chemist in the current context. We also note is known and much that remains strange about noble gas
that this behavior is also found for most simple multi- chemistry and the seemingly related chemistry of other
ply charged cations: it is only the presence of “lots” of nonmetals.
solvent, or an electrostatically stabilized counterion, that
makes Ca - and Fe -containing species normal chem-
3+
2+
D. Compounds of the Other Noble Gases
+
ical species and not exotic as are the subvalent Ca and
+
Fe seen in pulse-radiolysis experiments. What about neon, argon, krypton, and radon? We close
What about neutral helium-containing compounds? this section with brief notes about these other noble gases.
Atomic helium has been trapped inside dodecahedrane Pun intended, there is nothing new about neon. The chem-
and buckminsterfullerene, analogous to other “endohe- istry of argon has been revolutionalized by the formation
dral”speciestobediscussedlater.Aretheresultingspecies of H Ar F and its stability up to 27 K. The question of
C 20 H 20 He and C 60 He organohelium compounds, or are whether any argon compound will be isolable under am-
they better understood as related to the long-known noble bient conditions remains tantalizingly plausible, with still
+
gas clathrates? unknown [ArF] salts. The chemistry of krypton remains
dominated by the binary, divalent fluoride KrF 2 that has
the interesting property of being thermodynamically and
C. Xenon-Containing Species
kinetically stable with regard to the cleavage of one bond
We now jump to xenon. As mentioned earlier, xenon oc- but not both, while being immune to the latter for a variety
curs with oxidation states 0–8. [Isolable Xe(I) species of extrathermodynamic reasons. (In fact, this is not unique
seem not to have been made, despite the seeming to KrF 2 : formaldehyde has the same bond-cleaving char-
formation of XePtF 6 some 40 years ago that began acteristics and is only metastable relative to CO + H 2 .)
“conventional”—pardon the oxymoron—noble gas chem- And radon? We only wonder what exotic chemistry would
istry.] With the value of 8, xenon shares the highest ox- arise from radon (and its neighbors such as the heaviest al-
idation state value known in chemistry; only Os and Ru kali metal, francium, and heaviest halogen, astatine) were
form analogous isolable (but in these cases, nonexplo- it not so radioactive as to dissuade the general chemical
sive) tetraoxides. There is the value +1/2 as well, which community from its study.
is found in [Xe 2 ] . Having as precedents other [NgNg ] It is to be acknowledged now that a result being exotic

+
+
including[He 2 ] aswellasothernoble-gas“ion–molecule does not preclude its intelligibility nor even the absence
+
complexes” such as [ArN 2 ] and [XeCO] , [Xe 2 ] is a of “simple” explanations. Such is the case with the exis-
+
+
+
rather stable ion. What is different is that the dixenon com- tence of compounds of the heavier noble gases, krypton
pound lacks the electron voracity of the helium and the and xenon (and radon) as opposed to (at least, currently)
other species. As such, despite a smaller dissociation en- those of the lightest noble gases, helium and neon. Noble
ergy and thus weaker bond than [He 2 ] , [Xe 2 ] is capable gases do not want to accept electrons; their singly negative
+
+