Inchemistry,noble gas compounds arechemical compounds that include anelement from thenoble gases,group 8 or 18 of theperiodic table. Although the noble gases are generallyunreactive elements, many such compounds have been observed, particularly involving the elementxenon.
From the standpoint of chemistry, the noble gases may be divided into two groups:[citation needed] the relatively reactivekrypton (ionisation energy 14.0 eV), xenon (12.1 eV), andradon (10.7 eV) on one side, and the very unreactiveargon (15.8 eV),neon (21.6 eV), andhelium (24.6 eV) on the other. Consistent with this classification, Kr, Xe, and Rn form compounds that can be isolated in bulk at or nearstandard temperature and pressure, whereas He, Ne, Ar have been observed to form truechemical bonds usingspectroscopic techniques, but only when frozen into a noble gas matrix at temperatures of 40 K (−233 °C; −388 °F) or lower, in supersonic jets of noble gas, or under extremely high pressures with metals.
The heavier noble gases have moreelectron shells than the lighter ones. Hence, the outermost electrons are subject to ashielding effect from the inner electrons that makes them more easilyionized, since they are less strongly attracted to the positively-chargednucleus. This results in an ionization energy low enough to form stable compounds with the mostelectronegative elements,fluorine andoxygen, and even with less electronegative elements such asnitrogen andcarbon under certain circumstances.[1][2]
When the family of noble gases was first identified at the end of the nineteenth century, none of them were observed to form any compounds and so it was initially believed that they were allinert gases (as they were then known) which could not form compounds. With the development of atomic theory in the early twentieth century, their inertness was ascribed to a fullvalence shell ofelectrons which render them very chemically stable and nonreactive. All noble gases have fulls andp outerelectron shells (excepthelium, which has nop sublevel), and so do not formchemical compounds easily. Their highionization energy and almost zeroelectron affinity explain their non-reactivity.
In 1933,Linus Pauling predicted that the heavier noble gases would be able to form compounds withfluorine andoxygen. Specifically, he predicted the existence of krypton hexafluoride (KrF6) andxenon hexafluoride (XeF6), speculated thatXeF8 might exist as an unstable compound, and suggested thatxenic acid would formperxenate salts.[3][4] These predictions proved quite accurate, although subsequent predictions forXeF8 indicated that it would be not onlythermodynamically unstable, butkinetically unstable.[5] As of 2022,XeF8 has not been made, although the octafluoroxenate(VI) anion ([XeF8]2−) has been observed.
By 1960, no compound with a covalently bound noble gas atom had yet been synthesized.[6] The first published report, in June 1962, of a noble gas compound was byNeil Bartlett, who noticed that the highly oxidising compoundplatinum hexafluoride ionisedO2 toO+2. As the ionisation energy ofO2 toO+2 (1165 kJ mol−1) is nearly equal to the ionisation energy of Xe toXe+ (1170 kJ mol−1), he tried the reaction of Xe withPtF6. This yielded a crystalline product,xenon hexafluoroplatinate, whose formula was proposed to beXe+[PtF6]−.[4][7]It was later shown that the compound is actually more complex, containing both[XeF]+[PtF5]− and[XeF]+[Pt2F11]−.[8] Nonetheless, this was the first real compound of any noble gas.
The firstbinary noble gas compounds were reported later in 1962. Bartlett synthesizedxenon tetrafluoride (XeF4) by subjecting a mixture ofxenon and fluorine to high temperature.[9]Rudolf Hoppe, among other groups, synthesizedxenon difluoride (XeF2) by the reaction of the elements.[10]
Following the first successful synthesis ofxenon compounds, synthesis ofkrypton difluoride (KrF2) was reported in 1963.[11]
In this section, the non-radioactive noble gases are considered in decreasing order ofatomic weight, which generally reflects the priority of their discovery, and the breadth of available information for these compounds. The radioactive elements radon and oganesson are harder to study and are considered at the end of the section.
After the initial 1962 studies onXeF4 andXeF2, xenon compounds that have been synthesized include other fluorides (XeF6), oxyfluorides (XeOF2,XeOF4,XeO2F2,XeO3F2,XeO2F4) and oxides (XeO2,XeO3 andXeO4). Xenon fluorides react with several other fluorides to form fluoroxenates, such as sodium octafluoroxenate(VI) ((Na+)2[XeF8]2−),[citation needed] and fluoroxenonium salts, such as trifluoroxenonium hexafluoroantimonate ([XeF3]+[SbF6]−).[12]
In terms of other halide reactivity, short-livedexcimers of noble gashalides such asXeCl2 orXeCl are prepared in situ, and are used in the function ofexcimer lasers.[13]
Recently,[when?] xenon has been shown to produce a wide variety of compounds of the typeXeOnX2 wheren is 1, 2 or 3 and X is any electronegative group, such asCF3,C(SO2CF3)3,N(SO2F)2,N(SO2CF3)2,OTeF5,O(IO2F2), etc.; the range of compounds is impressive, similar to that seen with the neighbouring elementiodine, running into the thousands and involving bonds between xenon and oxygen, nitrogen, carbon, boron and even gold, as well asperxenic acid, several halides, and complex ions.[citation needed]
The compound[Xe2]+[Sb4F21]− contains a Xe–Xe bond, which is the longest element-element bond known (308.71 pm = 3.0871Å).[14] Short-livedexcimers ofXe2 are reported to exist as a part of the function ofexcimer lasers.[citation needed]
Krypton gas reacts with fluorine gas under extreme forcing conditions, formingKrF2 according to the following equation:
KrF2 reacts with strongLewis acids to form salts of the[KrF]+ and[Kr2F3]+cations.[11] The preparation ofKrF4 reported by Grosse in 1963, using the Claasen method, was subsequently shown to be a mistaken identification.[15]
Krypton compounds with other than Kr–F bonds (compounds with atoms other thanfluorine) have also been described.KrF2 reacts withB(OTeF5)3 to produce the unstable compound,Kr(OTeF5)2, with a krypton-oxygen bond. A krypton-nitrogen bond is found in thecation[H−C≡N−Kr−F]+, produced by the reaction ofKrF2 with[H−C≡N−H]+[AsF6]− below −50 °C.[16]
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The discovery ofHArF was announced in 2000.[17][18] The compound can exist in low temperatureargonmatrices for experimental studies, and it has also been studiedcomputationally.[18] Argon hydride ion[ArH]+ was obtained in the 1970s.[19]This molecular ion has also been identified in theCrab nebula, based on the frequency of its light emissions.[20]
There is a possibility that a solid salt of[ArF]+ could be prepared with[SbF6]− or[AuF6]− anions.[21][22]
The ions,Ne+,[NeAr]+,[NeH]+, and[HeNe]+ are known from optical and mass spectrometric studies. Neon also forms an unstable hydrate.[23] There is some empirical and theoretical evidence for a few metastablehelium compounds which may exist at very low temperatures or extreme pressures. The stable cation[HeH]+ was reported in 1925,[24] but was not considered a true compound since it is not neutral and cannot be isolated. In 2016 scientists created the helium compounddisodium helide (Na2He) which was the first helium compound discovered.[25]
Radon is not chemically inert, but its shorthalf-life (3.8 days for222Rn) and the high energy of its radioactivity make it difficult to investigate its only fluoride (RnF2), its reported oxide (RnO3), and their reaction products.[26]
All knownoganesson isotopes have even shorter half-lives in the millisecond range and no compounds are known yet,[27] although some have been predicted theoretically. It is expected to be even more reactive than radon, more like a normal element than a noble gas in its chemistry.[28]
Prior to 1962, the only isolated compounds of noble gases wereclathrates (including clathratehydrates); other compounds such ascoordination compounds were observed only by spectroscopic means.[4] Clathrates (also known as cage compounds) are compounds of noble gases in which they are trapped within cavities of crystal lattices of certain organic and inorganic substances. Ar, Kr, Xe and Ne[30] can form clathrates with crystallinehydroquinone. Kr and Xe can appear as guests in crystals ofmelanophlogite.[31]
Helium-nitrogen (He(N2)11) crystals have been grown at room temperature at pressures ca. 10 GPa in adiamond anvil cell.[32] Solid argon-hydrogen clathrate (Ar(H2)2) has the same crystal structure as theMgZn2Laves phase. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that theH2 molecules inAr(H2)2 dissociate above 175 GPa. A similarKr(H2)4 solid forms at pressures above 5 GPa. It has a face-centered cubic structure where krypton octahedra are surrounded by randomly oriented hydrogen molecules. Meanwhile, in solidXe(H2)8 xenon atoms form dimers insidesolid hydrogen.[29]
Coordination compounds such asAr·BF3 have been postulated to exist at low temperatures, but have never been confirmed.[citation needed]
Xenon is known to function as a metalligand. In addition to the charged[AuXe4]2+, xenon,krypton, andargon all reversibly bind to gaseousM(CO)5, where M=Cr, Mo, or W.P-block metals also bind noble gases: XeBeO has been observed spectroscopically and both XeBeS and FXeBO are predicted stable.[33]
Also, compounds such asWHe2 andHgHe2 were reported to have been formed by electron bombardment, but recent research has shown that these are probably the result of He beingadsorbed on the surface of the metal; therefore, these compounds cannot truly be considered chemical compounds.[citation needed]
Hydrates are formed by compressing noble gases in water, where it is believed that the water molecule, a strong dipole, induces a weak dipole in the noble gas atoms, resulting in dipole-dipole interaction. Heavier atoms are more influenced than smaller ones, henceXe·5.75H2O was reported to have been the most stable hydrate;[34] it has a melting point of 24 °C.[35] Thedeuterated version of this hydrate has also been produced.[36]
Noble gases can also formendohedral fullerene compounds where the noble gas atom is trapped inside afullerene molecule. In 1993, it was discovered that whenC60 is exposed to a pressure of around 3bar of He or Ne, the complexesHe@C60 andNe@C60 are formed.[37] Under these conditions, only about one out of every 650,000C60 cages was doped with ahelium atom; with higher pressures (3000 bar), it is possible to achieve a yield of up to 0.1%. Endohedral complexes withargon,krypton andxenon have also been obtained, as well as numerousadducts ofHe@C60.[38]
Most applications of noble gas compounds are either as oxidising agents or as a means to store noble gases in a dense form.Xenic acid is a valuable oxidising agent because it has no potential for introducing impurities—xenon is simply liberated as a gas—and so is rivalled only byozone in this regard.[4] Theperxenates are even more powerful oxidizing agents.[citation needed] Xenon-based oxidants have also been used for synthesizingcarbocations stable at room temperature, inSO2ClF solution.[39][non-primary source needed]
Stable salts of xenon containing very high proportions of fluorine by weight (such astetrafluoroammonium heptafluoroxenate(VI),[NF4][XeF7], and the relatedtetrafluoroammonium octafluoroxenate(VI)[NF4]2[XeF8]), have been developed as highly energetic oxidisers for use as propellants in rocketry.[40][non-primary source needed][41]
Xenon fluorides are good fluorinating agents.[42]
Clathrates have been used for separation of He and Ne from Ar, Kr, and Xe, and also for the transportation of Ar, Kr, and Xe.[citation needed] (For instance, radioactive isotopes of krypton and xenon are difficult to store and dispose, and compounds of these elements may be more easily handled than the gaseous forms.[4]) In addition, clathrates of radioisotopes may provide suitable formulations for experiments requiring sources of particular types of radiation; hence.85Kr clathrate provides a safe source ofbeta particles, while133Xe clathrate provides a useful source ofgamma rays.[43]
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