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Encyclopedia of Physical Science and Technology EN010K-480 July 16, 2001 17:22

460 Noble-Gas Chemistry

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of KrF and Kr 2 F are formed with the pentafluorides V. RADON COMPOUNDS
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of Group 15 elements and those of platinum and
−
−
−
−
+
+
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gold: KrF BiF , KrF SbF , KrF Sb 2 F ,Kr 2 F SbF , A. Radon Fluoride
6 6 11 3 6
+
−
−
+
−
−
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KrF AsF ,Kr 2 F AsF ,Kr 2 F PF , KrF Nb 2 F , KrF + All radon chemistry, of necessity, has been done at the
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3
6
6
6
−
−
−
−
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TaF , KrF Ta 2 F , KrF PtF , and KrF AuF . Unlike radiotracer level, precluding the possibility of obtain-
11
6
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their xenon analogues, the majority of these salts decom-
ing detailed structural support for the species that are
pose at or below room temperature, but at least three,
proposed.
−
−
−
KrF AsF , KrF SbF , and KrF Sb 2 F , are moderately
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6 6 11 When a mixture of trace amounts of radon-222 and flu-
stable at room temperature. As in XeF salts (see Section
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◦
orine gas are heated to approximately 400 C, a nonvolatile
III.E), KrF interacts rather strongly with the anion by
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fluoride is formed. The intense γ -radiation of millicurie
means of a fluorine bridge, e.g., KrF AsF [Kr F terminal , and curie amounts of radon provides the activation, allow-
−
+
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˚
˚
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1.765(2) A; Kr--F bridge , 2.131(2) A]. The Kr 2 F cation,
3 ing radon in such quantities to react spontaneously with
−
+
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like Xe 2 F , is V shaped, e.g., Kr 2 F AsF [Xe F terminal ,
3 3 6 gaseous fluorine at room temperature and with liquid flu-
˚
˚
1.792(8) A; Xe--F bridge , 2.055(7) A]. The KrF + cation
orine at −196 C. Radon is also oxidized by chlorine and
◦
ranks as the most powerful chemical oxidizer presently
bromine fluorides, IF 7 and NiF 2− in HF to give stable solu-
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known and is capable of oxidizing gaseous xenon to tions of radon fluoride. The products of these fluorination
+
+
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XeF ; gaseous oxygen to O ;NF 3 to NF ; and chlorine,
5 2 4 reactions have not been analyzed because of their small
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bromine, and iodine pentafluorides to the ClF , BrF , and
6 6 masses and intense radioactivity. It has nevertheless been
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IF cations, respectively. Adducts with the weak fluoride
6 possible to deduce that radon forms a difluoride, RnF 2 ,
ion acceptors CrOF 4 , MoOF 4 , and WOF 4 are known in
and derivatives of the difluoride by comparing reactions
which KrF 2 interacts with the metal center by formation
of radon with those of krypton and xenon. Electromigra-
of asymmetric Kr F--M bridges (M = Cr, Mo, W), e.g.,
tion and ion exchange studies show that ionic radon is
F Kr F--MOF 4 ,F Kr F--MoOF 4 (MoOF 4 ) 2 .
present in many of these solutions and is believed to be
2+
+
RnF and Rn . The chemical behavior of radon is sim-
ilar to that of a metal fluoride and is consistent with its
C. Other Krypton Derivatives position in the Periodic Table as a metalloid element.
Despite the strong oxidizing properties of the KrF cation,
+
it has been shown to behave as a Lewis acid towards a
B. The Fluorocations of Radon
limited number of Lewis bases at low temperatures. These
bases are resistant to oxidation by the strongly oxidizing Radon reacts at room temperature with solid oxidants
+
−
+
+
−
−
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KrF cation and, as in the case of the XeF cation, they such as O SbF ,O Sb 2 F ,N 2 F SbF , and BrF BiF −
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2
2
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6
6
6
2
have first adiabatic ionization potentials that are greater to form nonvolatile complex salts, which are believed to
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−
+
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than or comparable to the estimated electron affinity of be RnF SbF , RnF Sb 2 F , and RnF BiF , by analogy
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+
the KrF cation (13.2 eV). The Lewis acid-base cations with krypton xenon, which form the well-characterized
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−
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◦
are all thermally unstable above ca. −40 C and consist of salts NgF SbF , NgF Sb 2 F , and NgF BiF − where
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+
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HC N KrF ,F 3 CC N KrF ,F 3 CCF 2 C N KrF , Ng = Kr or Xe (see Section III.E and IV.B).
+
and n-F 3 CF 2 CF 2 C N KrF , all having AsF − as the
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counteranion. These cations comprise the only examples
ofkryptonbondedtonitrogen.ThecompoundKr(OTeF 5 ) 2 VI. PROSPECTS FOR ARGON
provides the only reported example of a compound in COMPOUNDS
which krypton is bonded to oxygen. The existence of the
+
KrCH cation has been established in the gas phase by The photolysis of HF in a solid Ar matrix gives Ar fluo-
3
ion cyclotron resonance spectroscopy. The Kr C bond rohydride (HArF), which has been identified by infrared
energy of the KrCH + cation has been estimated to be spectroscopy. Theoretical calculations indicate that HArF
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199.6 ± 10.5 kJ mol −1 and is considerably more stable is intrinsically stable, owing to significant ionic and cova-
than the Kr F bonds of KrF 2 (mean thermochemical bond lent contributions to its bonding, thus confirming compu-
−1
−1
energy of 48.9 kJ mol ) and KrF (∼155 kJ mol ). No tational predictions that Ar should form a stable hydride
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compounds in which krypton is bonded to elements other species with properties similar to those of the analogous
than fluorine, oxygen, and nitrogen have been isolated. matrix-isolated Xe and Kr compounds.
The nonexistence of simple oxides or oxofluorides is Theoretical calculations indicate that argon difluoride
consistent with the lack of a higher oxidation state may be unstable but that the ArF cation will be stable
+
of krypton. in the presence of a suitable oxidatively resistant anion.