Noble gas compound

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 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 () and xenon hexafluoride (), speculated that might exist as an unstable compound, and suggested that xenic acid would form perxenate salts. 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, although these were ignored by the broader chemistry community. By 1960, no compound with a covalently bound noble gas atom had yet been synthesized. 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 oxygen| to dioxygenyl|. As the ionisation energy of to (1165 kJ mol−1) is nearly equal to the ionisation energy of Xe to (1170 kJ mol−1), he tried the reaction of Xe with . This yielded a crystalline product, xenon hexafluoroplatinate, whose formula was proposed to be . It was later shown that the compound is actually more complex, containing both and . 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 (). Meanwhile, Rudolf Hoppe, among other groups, synthesized xenon difluoride () from the elements. Pauling et al's predictions thus proved quite accurate, although appears not only thermodynamically, but kinetically unstable. As of 2022, has not been made, and only the octafluoroxenate(VI) anion (Nitrosonium octafluoroxenate(VI)|) observed. Following the first successful synthesis of xenon compounds, synthesis of krypton difluoride () was reported in 1963. ==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 Xenon tetrafluoride| and Xenon difluoride|, xenon compounds that have been synthesized include other fluorides (xenon hexafluoride|), oxyfluorides (Xenon oxydifluoride|, Xenon oxytetrafluoride|, Xenon dioxydifluoride|, , ) and oxides (xenon dioxide|, xenon trioxide| and xenon tetroxide|). Xenon fluorides react with several other fluorides to form fluoroxenates, such as sodium octafluoroxenate(VI) (), and fluoroxenonium salts, such as trifluoroxenonium hexafluoroantimonate (). In terms of other halide reactivity, short-lived excimers of noble gas halides such as xenon dichloride| or XeCl are prepared in situ, and are used in the function of excimer lasers. Recently, xenon has been shown to produce a wide variety of compounds of the type {{chem2|XeO_{n}X2}} where n is 1, 2 or 3 and X is any electronegative group, such as , Triflidic acid|, , Bistriflimide|, Teflate|, , 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. The compound contains a Xe–Xe bond, which is the longest element-element bond known (308.71 pm = 3.0871 Å). Short-lived excimers of are reported to exist as a part of the function of excimer lasers. Krypton compounds Krypton gas reacts with fluorine gas under extreme forcing conditions, forming Krypton difluoride| according to the following equation: : reacts with strong Lewis acids to form salts of the and cations. Krypton compounds with other than Kr–F bonds (compounds with atoms other than fluorine) have also been described. reacts with to produce the unstable compound, , with a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation , produced by the reaction of with below −50 °C. Argon compounds Reported in 1970, neutral argon monohydride (ArH) was the first discovered hydride of a noble gas. It is unstable in its ground state, but can form stable Rydberg molecules. The argon hydride ion (argonium) was obtained in the 1970s. This molecular ion has also been identified in the Crab Nebula, based on the frequency of its light emissions. The discovery of HArF was announced in 2000. The compound can exist in low temperature argon matrices for experimental studies, and it has also been studied computationally. Various argon-nitrogen cations have been detected, such as [ArNH]+, [ArN2]+, [ArHN2]+, [Ar(N2)2]+, and [ArN2O]+. 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) and ArBeS (from reaction of beryllium with hydrogen sulfide trapped in an argon matrix at 4 K). Neon and helium compounds The ions , , , and are known from optical and mass spectrometric studies. 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 helium hydride ion| was reported in 1925, 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 () which was the first helium compound discovered. The compound does not have true bonds between the helium and sodium atoms, but instead the helium atoms stabilize the solid lattice. 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 (), its reported oxide (), and their reaction products. All known oganesson isotopes have even shorter half-lives in the millisecond range and no compounds are known yet, 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. ==Reports prior to xenon hexafluoroplatinate and xenon tetrafluoride ==

Clathrates . Ruby was added for pressure measurement. can form clathrates with crystalline hydroquinone. Kr and Xe can appear as guests in crystals of melanophlogite. Neon forms a clathrate hydrate stable at high pressure and low temperature. Helium-nitrogen () crystals have been grown at room temperature at pressures ca. 10 GPa in a diamond anvil cell. Solid argon-hydrogen clathrate () has the same crystal structure as the Laves phase. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the molecules in dissociate above 175 GPa. A similar 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 xenon atoms form dimers inside solid hydrogen. Coordination compounds Coordination compounds such as have been postulated to exist at low temperatures, but have never been confirmed. Xenon is known to function as a metal ligand. In addition to the charged tetraxenonogold(II)|[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. Also, compounds such as and 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. 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 was reported to have been the most stable hydrate; it has a melting point of 24 °C. The deuterated version of this hydrate has also been produced. ==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 is exposed to a pressure of around 3 bar of He or Ne, the complexes and are formed. Under these conditions, only about one out of every 650,000 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 . ==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. Stable salts of xenon containing very high proportions of fluorine by weight (such as tetrafluoroammonium heptafluoroxenate(VI), , and the related tetrafluoroammonium octafluoroxenate(VI) ), have been developed as highly energetic oxidisers for use as propellants in rocketry. Xenon fluorides are good fluorinating agents. 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. (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. ==References==