In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 8 or 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particularly involving the element xenon.

From the standpoint of chemistry, the noble gases may be divided into two groups:[citation needed] the relatively reactive krypton (ionisation energy 14.0 eV), xenon (12.1 eV), and radon (10.7 eV) on one side, and the very unreactive argon (15.8 eV), neon (21.6 eV), and helium (24.6 eV) on the other. Consistent with this classification, Kr, Xe, and Rn form compounds that can be isolated in bulk at or near standard temperature and pressure, whereas He, Ne, Ar have been observed to form true chemical bonds using spectroscopic 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 more electron shells than the lighter ones. Hence, the outermost electrons are subject to a shielding effect from the inner electrons that makes them more easily ionized, since they are less strongly attracted to the positively-charged nucleus. This results in an ionization energy low enough to form stable compounds with the most electronegative elements, fluorine and oxygen, and even with less electronegative elements such as nitrogen and carbon under certain circumstances.[1][2]

History and background

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 all inert gases (as they were then called) which could not form compounds. With the development of atomic theory in the early twentieth century, their inertness was ascribed to a full valence shell of electrons which render them very chemically stable and nonreactive. All noble gases have full s and p outer electron shells (except helium, which has no p sublevel), and so do not form chemical compounds easily. Their high ionization energy and negative electron affinity explain their non-reactivity.

In 1933, Linus Pauling predicted that the heavier noble gases would be able to form compounds with fluorine and oxygen. Specifically, he predicted the existence of krypton hexafluoride (KrF6) and xenon hexafluoride (XeF6), speculated that XeF8 might exist as an unstable compound, and suggested that xenic acid would form perxenate salts.[3][4] Quantum-chemical calculations subsequently supported Pauling's speculation, suggesting that bonding in a (then-hypothetical) noble gas compound would resemble bonding in the well-known trihalogenide ions,[5] although these were ignored by the broader chemistry community.[6] By 1960, no compound with a covalently bound noble gas atom had yet been synthesized.[7]

In June 1962, Neil Bartlett gave the first creditable report of a noble gas compound. Bartlett had noticed that the highly oxidising compound platinum hexafluoride ionised O2 to O2+. As the ionisation energy of O2 to O2+ (1165 kJ mol−1) is nearly equal to the ionisation energy of Xe to Xe+ (1170 kJ mol−1), he tried the reaction of Xe with PtF6. This yielded a crystalline product, xenon hexafluoroplatinate, whose formula was proposed to be Xe+[PtF6]-.[4][8] It was later shown that the compound is actually more complex, containing both [XeF]+[PtF5]- and [XeF]+[Pt2F11]-.[9] Nonetheless, this was the first real compound of any noble gas.

The psychological barrier broken, the first binary noble gas compounds appeared later that year. Bartlett subjected a mixture of xenon and fluorine to high temperature, obtaining xenon tetrafluoride (XeF4).[10] Meanwhile, Rudolf Hoppe, among other groups, synthesized xenon difluoride (XeF2) from the elements.[11] Pauling et al's predictions thus proved quite accurate, although XeF8 appears not only thermodynamically, but kinetically unstable.[12] As of 2022, XeF8 has not been made, and only the octafluoroxenate(VI) anion ([XeF8](2-)) observed.

Following the first successful synthesis of xenon compounds, synthesis of krypton difluoride (KrF2) was reported in 1963.[13]

True noble gas compounds

In this section, the non-radioactive noble gases are considered in decreasing order of atomic 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.

Xenon compounds

Main article: Xenon compounds

After the initial 1962 studies on XeF4 and XeF2, xenon compounds that have been synthesized include other fluorides (XeF6), oxyfluorides (XeOF2, XeOF4, XeO2F2, XeO3F2, XeO2F4) and oxides (XeO2, XeO3 and XeO4). 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]-).[14]

In terms of other halide reactivity, short-lived excimers of noble gas halides such as XeCl2 or XeCl are prepared in situ, and are used in the function of excimer lasers.[15]

Recently,[when?] xenon has been shown to produce a wide variety of compounds of the type XeO_{n}X2 where n is 1, 2 or 3 and X is any electronegative group, such as CF3, 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 element iodine, running into the thousands and involving bonds between xenon and oxygen, nitrogen, carbon, boron and even gold, as well as perxenic 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 = 3.0871 Å).[16] Short-lived excimers of Xe2 are reported to exist as a part of the function of excimer lasers.[citation needed]

Krypton compounds

Main article: Krypton#Chemistry

Krypton gas reacts with fluorine gas under extreme forcing conditions, forming KrF2 according to the following equation:

Kr + F2 → KrF2

KrF2 reacts with strong Lewis acids to form salts of the [KrF]+ and [Kr2F3]+ cations.[13] The preparation of KrF4 reported by Grosse in 1963, using the Claasen method, was subsequently shown to be a mistaken identification.[17]

Krypton compounds with other than Kr–F bonds (compounds with atoms other than fluorine) have also been described. KrF2 reacts with B(OTeF5)3 to produce the unstable compound, Kr(OTeF5)2, with a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation [H\sC\tN\sKr\sF]+, produced by the reaction of KrF2 with [H\sC\tN\sH]+[AsF6]− below −50 °C.[18]

Argon compounds

Main article: Argon compounds

Reported in 1970, neutral argon monohydride (ArH) was the first discovered hydride of a noble gas.[19] It is unstable in its ground state, but can form stable Rydberg molecules.[20][21] The argon hydride ion [ArH]+ (argonium) was obtained in the 1970s.[22] This molecular ion has also been identified in the Crab Nebula, based on the frequency of its light emissions.[23]

The discovery of HArF was announced in 2000.[24][25] The compound can exist in low temperature argon matrices for experimental studies, and it has also been studied computationally.[25] Hexafluoroantimonate [SbF6]− and hexafluoroaurate [AuF6]− salts of [ArF]+ have been predicted but not yet isolated.[26][27]

Various argon-nitrogen cations have been detected, such as [ArNH]+,[28] [ArN2]+,[29] [ArHN2]+,[30] [Ar(N2)2]+,[31] and [ArN2O]+.[32][33] These are often linear species (e.g. [ArHN2]+ is Ar−H−N−N and [Ar(N2)2]+ is N=N−Ar+−N=N).

Argon-beryllium compounds have been reported, such as ArBeO (from reaction of beryllium atoms with oxygen in a solid argon matrix)[34] and ArBeS (from reaction of beryllium with hydrogen sulfide trapped in an argon matrix at 4 K).[35]

Neon and helium compounds

Main articles: Helium compounds and Neon compounds

The ions Ne+, [NeAr]+, [NeH]+, and [HeNe]+ are known from optical and mass spectrometric studies.[36] There is some empirical and theoretical evidence for a few metastable helium compounds which may exist at very low temperatures or extreme pressures. The stable cation [HeH]+ was reported in 1925,[37] but was not considered a true compound since it is not neutral and cannot be isolated. In 2016 scientists created the helium compound disodium helide (Na2He) which was the first helium compound discovered.[38] The compound does not have true bonds between the helium and sodium atoms, but instead the helium atoms stabilize the solid lattice.[39]

Radon and oganesson compounds

Main articles: Radon compounds and Oganesson#Predicted compounds

Radon is not chemically inert, but its short half-life (3.8 days for 222Rn) and the high energy of its radioactivity make it difficult to investigate its only fluoride (RnF2), its reported oxide (RnO3), and their reaction products.[40]

All known oganesson isotopes have even shorter half-lives in the millisecond range and no compounds are known yet,[41] 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.[42]

Reports prior to xenon hexafluoroplatinate and xenon tetrafluoride

Clathrates

Prior to 1962, the only isolated compounds of noble gases were clathrates (including clathrate hydrates); other compounds such as coordination 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[43] can form clathrates with crystalline hydroquinone. Kr and Xe can appear as guests in crystals of melanophlogite.[44] Neon forms a clathrate hydrate stable at high pressure and low temperature.[45]

Helium-nitrogen (He(N2)11) crystals have been grown at room temperature at pressures ca. 10 GPa in a diamond anvil cell.[46] Solid argon-hydrogen clathrate (Ar(H2)2) has the same crystal structure as the MgZn2 Laves phase. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the H2 molecules in Ar(H2)2 dissociate above 175 GPa. A similar Kr(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 solid Xe(H2)8 xenon atoms form dimers inside solid hydrogen.[47]

Coordination compounds

Coordination compounds such as Ar*BF3 have been postulated to exist at low temperatures, but have never been confirmed.[citation needed]

Xenon is known to function as a metal ligand. In addition to the charged [AuXe4]2+, xenon, krypton, and argon all reversibly bind to gaseous M(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.[48]

Also, compounds such as WHe2 and HgHe2 were reported to have been formed by electron bombardment, but recent research has shown that these are probably the result of He being adsorbed on the surface of the metal; therefore, these compounds cannot truly be considered chemical compounds.[citation needed]

Hydrates

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, hence Xe*5.75H2O was reported to have been the most stable hydrate;[49] it has a melting point of 24 °C.[50] The deuterated version of this hydrate has also been produced.[51]

Fullerene adducts

Main article: Non-metal doped fullerenes

Noble gases can also form endohedral fullerene compounds where the noble gas atom is trapped inside a fullerene molecule. In 1993, it was discovered that when C60 is exposed to a pressure of around 3 bar of He or Ne, the complexes He@C60 and Ne@C60 are formed.[52] Under these conditions, only about one out of every 650,000 C60 cages was doped with a helium atom; with higher pressures (3000 bar), it is possible to achieve a yield of up to 0.1%. Endohedral complexes with argon, krypton and xenon have also been obtained, as well as numerous adducts of He@C60.[53]

Applications

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 by ozone in this regard.[4] The perxenates are even more powerful oxidizing agents.[citation needed] Xenon-based oxidants have also been used for synthesizing carbocations stable at room temperature, in SO2ClF solution.[54][non-primary source needed]

Stable salts of xenon containing very high proportions of fluorine by weight (such as tetrafluoroammonium heptafluoroxenate(VI), [NF4][XeF7], and the related tetrafluoroammonium octafluoroxenate(VI) [NF4]2[XeF8]), have been developed as highly energetic oxidisers for use as propellants in rocketry.[55][non-primary source needed] [56]

Xenon fluorides are good fluorinating agents.[57]

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 of beta particles, while 133Xe clathrate provides a useful source of gamma rays.[58]

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Resources

  • Khriachtchev, Leonid; Räsänen, Markku; Gerber, R. Benny (2009). "Noble-Gas Hydrides: New Chemistry at Low Temperatures". Accounts of Chemical Research. 42 (1): 183–91. doi:10.1021/ar800110q. PMID 18720951