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Noble-Gas Chemistry 459
˚
by X-ray crystallography [Xe C, 2.11(1) A; Xe Cl, troscopy in the solid state. The identification and char-
˚
2.816(3) A]. acterization of such species provide direction and in-
Hydrogen-containing (polyfluorocycloalken-1-yl)xen- centive to synthetic chemists who pursue the synthe-
on(II) salts (2-H-hexafluoro-1,4-cyclohexadien-1-yl) xen- ses of noble-gas compounds that contain novel bonds
on(II)and(2-H-octafluorocyclohexen-1-yl)xenon(II)hex- to other elements. Photolysis of H 2 S in a Xe matrix
afluoro-arsenates and tetrafluoroborates have been at 7.5 K has afforded HXeSH, the first and only ex-
obtained together with their perfluorinated analogs ample of an Xe S bond. The structure was determined
on reacting the corresponding (2,3,4,5-tetrafluoro- by infrared spectroscopy. The gaseous trifluorosilylxenon
+
phenyl)xenon(II) salts with XeF 2 in anhydrous HF. An cation F 3 SiXe , a species with an Si Xe bond, was ob-
example of an acyclic alkenylxenon(II) compound, trifluo- tained under mass spectrometric conditions from the nu-
−
+
rovinylxenon(II) tetrafluoroborate, F 2 C C(F)Xe BF ,is cleophilic displacement of HF by Xe from protonated
4
SiF 4 .
known; it was synthesized from XeF 2 and F 2 C C(F)BF 2
and was characterized by 13 C, 19 F, and 129 Xe NMR
spectroscopy.
Examples of alkynyl xenonium tetrafluoroborates are IV. KRYPTON COMPOUNDS
known and are prepared by reaction of lithium acetylides
A. Krypton Difluoride
or triphenylsilyl acetylenes with XeF 2 and BF 3 ·O(CH 3 ) 2
at low temperatures (−78 to −40 C) in CH 2 Cl 2 solvent.
◦
The known alkynyl derivatives are (CH 3 ) 3 C C C Krypton is the lightest of the noble gases that forms
−
−
Xe BF , (CH 3 ) 3 Si C C Xe BF ,CH 3 CH 2 C C isolable chemical compounds in bulk amounts. The
+
+
4 4
−
−
+
+
Xe BF ,andCH 3 CH 2 CH 2 C C Xe BF .Allofthese simplest of these compounds is KrF 2 , a colorless crys-
4 4
◦
◦
species decompose in solution or as solids below 0 C. talline solid which can be sublimed under vacuum at 0 C
The first organoxenon(IV) compound, [C 6 F 5 XeF 2 ] but is thermodynamically unstable and slowly decom-
[BF 4 ], was prepared by reaction of C 6 F 5 BF 2 with XeF 4 in poses to the elements at ambient temperatures. It can, how-
◦
◦
CH 2 Cl 2 at −55 C and is obtained as a yellow solid. The ever, be stored for indefinite periods of time at −78 C.
cation geometry is analogous to that of the T-shaped XeF + The KrF 2 molecule has been shown, like XeF 2 ,tobe
3
cation in which the unique equatorial fluorine is replaced linear in the gas phase, in the solid state, and in solu-
by the C 6 F 5 group. tion. The standard enthalpy of formation (derived from
+
The existence of the XeCH cation has been established calorimetric measurements of the gaseous compound at
3
−1
◦
in the gas phase. The Xe C bond energy of the XeCH + 93 C) is 60.2 kJ mol . Consistent with its thermody-
3
cation has been estimated to be 180 ± 33 kJ mol −1 and, namic instability, krypton difluoride is a powerful oxida-
more recently, 231 ± 10 kJ mol −1 by ion cyclotron reso- tive fluorinating agent and is capable of oxidizing xenon
nance (cf. XeF 2 and XeF ; the respective bond energies to XeF 6 and gold to AuF 5 . The heat of atomization for
+
−1
are 132 and 201 kJ mol ). The compound Xe(CF 3 ) 2 is KrF 2 is only 97.8 kJ mol −1 and is substantially less than
−1
reported to be a waxy white solid having a half-life of that of F 2 (157.7 ± 0.4 kJ mol ), making it a better low-
ca. 30 min at room temperature. The synthesis involved temperature source of fluorine atoms and an aggressive
the addition of XeF 2 to a trifluoromethyl plasma, but the fluorinating agent at even low temperatures. Although the
characterization of this compound is limited and has not first krypton compound to be prepared was described as
been independently confirmed. the tetrafluoride, the properties ascribed to this material
have been shown to be those of the difluoride. No other
molecular fluoride of krypton is known, so all krypton
4. Xenon Bonded to Other Elements
compounds are derived from KrF 2 . Krypton difluoride
−
2+
The first metal–xenon compound, AuXe (Sb 2 F ) 2 , con- is a powerful oxidizing agent and is capable of oxidiz-
4 11
tains gold–xenon bonds and was prepared by reduction of ing and fluorinating xenon gas to XeF 6 and gold metal to
AuF 3 with elemental xenon. The square planar AuXe 2+ AuF 5 .
4
˚
cation has an Au Xe bond length of 2.74 A. The bonding
between gold and xenon is of the σ-donor type, resulting
B. The Fluorocations of Krypton
in a charge of approximately 0.4 per xenon atom.
and Their Salts
There are a number of novel xenon-element bonds
+
+
which have been characterized in species that have not The KrF and Kr 2 F cations are formed in reactions of
3
been obtained in bulk quantities but have been identi- KrF 2 with strong and weak fluoride ion acceptors to give
fied by mass spectrometry or by matrix-isolation spec- complex salts that are analogous to those of XeF 2 . Salts