Noble gas compound

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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.

Contents

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 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 ( Kr F 6) 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 XeOnX2 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

Kr(H2)4 and H2 solids formed in a diamond anvil cell. Ruby was added for pressure measurement. Krypton hydride crystal.jpg
Kr(H2)4 and H2 solids formed in a diamond anvil cell. Ruby was added for pressure measurement.
Structure of Kr(H2)4. Krypton octahedra (green) are surrounded by randomly oriented hydrogen molecules. Krypton hydride structure.png
Structure of Kr(H2)4. Krypton octahedra (green) are surrounded by randomly oriented hydrogen molecules.

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

Structure of a noble-gas atom caged within a buckminsterfullerene ( C60) molecule. Endohedral fullerene.png
Structure of a noble-gas atom caged within a buckminsterfullerene (C60) molecule.

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]

Related Research Articles

<span class="mw-page-title-main">Noble gas</span> Group of low-reactive, gaseous chemical elements

The noble gases are the members of group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some cases, oganesson (Og). Under standard conditions, the first six of these elements are odorless, colorless, monatomic gases with very low chemical reactivity and cryogenic boiling points. The properties of the seventh, unstable, element, Og, are uncertain.

<span class="mw-page-title-main">Xenon</span> Chemical element with atomic number 54 (Xe)

Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized.

<span class="mw-page-title-main">Xenon hexafluoroplatinate</span> Chemical compound

Xenon hexafluoroplatinate is the product of the reaction of platinum hexafluoride with xenon, in an experiment that proved the chemical reactivity of the noble gases. This experiment was performed by Neil Bartlett at the University of British Columbia, who formulated the product as "Xe+[PtF6]−", although subsequent work suggests that Bartlett's product was probably a salt mixture and did not in fact contain this specific salt.

<span class="mw-page-title-main">Excimer laser</span> Type of ultraviolet laser important in chip manufacturing and eye surgery

An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and micromachining.

<span class="mw-page-title-main">Xenon hexafluoride</span> Chemical compound

Xenon hexafluoride is a noble gas compound with the formula XeF6. It is one of the three binary fluorides of xenon that have been studied experimentally, the other two being XeF2 and XeF4. All known are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinating agent of the series. It is a colorless solid that readily sublimes into intensely yellow vapors.

<span class="mw-page-title-main">Platinum hexafluoride</span> Chemical compound

Platinum hexafluoride is the chemical compound with the formula PtF6, and is one of seventeen known binary hexafluorides. It is a dark-red volatile solid that forms a red gas. The compound is a unique example of platinum in the +6 oxidation state. With only four d-electrons, it is paramagnetic with a triplet ground state. PtF6 is a strong fluorinating agent and one of the strongest oxidants, capable of oxidising xenon and O2. PtF6 is octahedral in both the solid state and in the gaseous state. The Pt-F bond lengths are 185 picometers.

<span class="mw-page-title-main">Xenon difluoride</span> Chemical compound

Xenon difluoride is a powerful fluorinating agent with the chemical formula XeF
2, and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture-sensitive. It decomposes on contact with water vapor, but is otherwise stable in storage. Xenon difluoride is a dense, colourless crystalline solid.

Xenon compounds are compounds containing the element xenon (Xe). After Neil Bartlett's discovery in 1962 that xenon can form chemical compounds, a large number of xenon compounds have been discovered and described. Almost all known xenon compounds contain the electronegative atoms fluorine or oxygen. The chemistry of xenon in each oxidation state is analogous to that of the neighboring element iodine in the immediately lower oxidation state.

<span class="mw-page-title-main">Xenon oxytetrafluoride</span> Chemical compound

Xenon oxytetrafluoride is an inorganic chemical compound. It is an unstable colorless liquid with a melting point of −46.2 °C that can be synthesized by partial hydrolysis of XeF
6, or the reaction of XeF
6 with silica or NaNO
3:

<span class="mw-page-title-main">Krypton difluoride</span> Chemical compound

Krypton difluoride, KrF2 is a chemical compound of krypton and fluorine. It was the first compound of krypton discovered. It is a volatile, colourless solid at room temperature. The structure of the KrF2 molecule is linear, with Kr−F distances of 188.9 pm. It reacts with strong Lewis acids to form salts of the KrF+ and Kr
2F+
3 cations.

<span class="mw-page-title-main">Dioxygenyl</span> Chemical compound

The dioxygenyl ion, O+
2, is a rarely-encountered oxycation in which both oxygen atoms have a formal oxidation state of +⁠1/2⁠. It is formally derived from oxygen by the removal of an electron:

<span class="mw-page-title-main">Krypton</span> Chemical element with atomic number 36 (Kr)

Krypton is a chemical element; it has symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps. Krypton is chemically inert.

A hexafluoride is a chemical compound with the general formula QXnF6, QXnF6m−, or QXnF6m+. Many molecules fit this formula. An important hexafluoride is hexafluorosilicic acid (H2SiF6), which is a byproduct of the mining of phosphate rock. In the nuclear industry, uranium hexafluoride (UF6) is an important intermediate in the purification of this element.

Fluorine forms a great variety of chemical compounds, within which it always adopts an oxidation state of −1. With other atoms, fluorine forms either polar covalent bonds or ionic bonds. Most frequently, covalent bonds involving fluorine atoms are single bonds, although at least two examples of a higher order bond exist. Fluoride may act as a bridging ligand between two metals in some complex molecules. Molecules containing fluorine may also exhibit hydrogen bonding. Fluorine's chemistry includes inorganic compounds formed with hydrogen, metals, nonmetals, and even noble gases; as well as a diverse set of organic compounds. For many elements the highest known oxidation state can be achieved in a fluoride. For some elements this is achieved exclusively in a fluoride, for others exclusively in an oxide; and for still others the highest oxidation states of oxides and fluorides are always equal.

<span class="mw-page-title-main">Xenon fluoride nitrate</span> Chemical compound

Xenon fluoride nitrate, also known as fluoroxenonium nitrate, is the chemical compound with formula FXeONO2.

Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly considered that helium compounds cannot exist at all, or at least under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero. The helium atom is small with the radius of the outer electron shell at 0.29 Å. Helium is a very hard atom with a Pearson hardness of 12.3 eV. It has the lowest polarizability of any kind of atom, however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point of any known substance.

Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have also been predicted to be stable, but are yet to be discovered in nature. Neon has been shown to crystallize with other substances and form clathrates or Van der Waals solids.

Argon compounds, the chemical compounds that contain the element argon, are rarely encountered due to the inertness of the argon atom. However, compounds of argon have been detected in inert gas matrix isolation, cold gases, and plasmas, and molecular ions containing argon have been made and also detected in space. One solid interstitial compound of argon, Ar1C60 is stable at room temperature. Ar1C60 was discovered by the CSIRO.

<span class="mw-page-title-main">Xenon dibromide</span> Chemical compound

Xenon dibromide is an unstable chemical compound with the chemical formula XeBr2. It was only produced by the decomposition of iodine-129:

<span class="mw-page-title-main">Radon compounds</span>

Radon compounds are chemical compounds formed by the element radon (Rn). Radon is a noble gas, i.e. a zero-valence element, and is chemically not very reactive. The 3.8-day half-life of radon-222 makes it useful in physical sciences as a natural tracer. Because radon is a gas under normal circumstances, and its decay-chain parents are not, it can readily be extracted from them for research.

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