In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 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 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 known) 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 almost zero 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] These predictions proved quite accurate, although subsequent predictions for XeF₈ indicated that it would be not only thermodynamically unstable, but kinetically 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 by Neil Bartlett, who noticed that the highly oxidising compound platinum hexafluoride ionised O₂ to O+2. As the ionisation energy of O₂ to O+2 (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][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 first binary noble gas compounds were reported later in 1962. Bartlett synthesized xenon tetrafluoride (XeF₄) by subjecting a mixture of xenon and fluorine to high temperature.^[9] Rudolf Hoppe, among other groups, synthesized xenon difluoride (XeF₂) by the reaction of the elements.^[10]

Following the first successful synthesis of xenon compounds, synthesis of krypton difluoride (KrF₂) was reported in 1963.^[11]

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

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]⁻).^[12]

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.^[13]

Recently,^[when?] xenon has been shown to produce a wide variety of compounds of the type XeO_nX₂ 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).^[14] Short-lived excimers of Xe₂ are reported to exist as a part of the function of excimer lasers.^{[citation needed]}

Krypton compounds

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.^[11] The preparation of KrF₄ 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 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−C≡N−Kr−F]⁺, produced by the reaction of KrF₂ with [H−C≡N−H]⁺[AsF₆]⁻ below −50 °C.^[16]

Argon compounds

The discovery of HArF was announced in 2000.^[17][18] The compound can exist in low temperature argon matrices for experimental studies, and it has also been studied computationally.^[18] Argon hydride ion [ArH]⁺ was obtained in the 1970s.^[19] This molecular ion has also been identified in the Crab 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]

Neon and helium compounds

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 metastable helium 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 compound disodium helide (Na₂He) which was the first helium compound discovered.^[25]

Radon and oganesson 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.^[26]

All known oganesson 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]

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^[30] can form clathrates with crystalline hydroquinone. Kr and Xe can appear as guests in crystals of melanophlogite.^[31]

Helium-nitrogen (He(N₂)₁₁) crystals have been grown at room temperature at pressures ca. 10 GPa in a diamond anvil cell.^[32] 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.^[29]

Coordination compounds

Coordination compounds such as Ar·BF₃ have been postulated to exist at low temperatures, but have never been confirmed.^{[citation needed]} 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;^[33] it has a melting point of 24 °C.^[34] The deuterated version of this hydrate has also been produced.^[35]

Fullerene adducts

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.^[36] 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.^[37]

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.^{[38][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.^{[39][non-primary source needed] [40]}

Xenon fluorides are good fluorinating agents.^[41]

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.^[42]