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.

Argon ionises at 15.76 eV, which is higher than hydrogen, but lower than helium, neon or fluorine.^[1] Molecules containing argon can be van der Waals molecules held together very weakly by London dispersion forces. Ionic molecules can be bound by charge induced dipole interactions. With gold atoms there can be some covalent interaction.^[2] Several boron-argon bonds with significant covalent interactions have been also reported.^[3][4] Experimental methods used to study argon compounds have included inert gas matrices, infrared spectroscopy to study stretching and bending movements, microwave spectroscopy and far infrared to study rotation, and also visible and ultraviolet spectroscopy to study different electronic configurations including excimers. Mass spectroscopy is used to study ions.^[5] Computation methods have been used to theoretically compute molecule parameters, and predict new stable molecules. Computational ab initio methods used have included CCSD(T), MP2 (Møller–Plesset perturbation theory of the second order), CIS and CISD. For heavy atoms, effective core potentials are used to model the inner electrons, so that their contributions do not have to be individually computed. More powerful computers since the 1990s have made this kind of in silico study much more popular, being much less risky and simpler than an actual experiment.^[5] This article is mostly based on experimental or observational results.

The argon fluoride laser is important in photolithography of silicon chips. These lasers make a strong ultraviolet emission at 192 nm.^[6]

Argonium

Argonium (ArH⁺) is an ion combining a proton and an argon atom. It is found in interstellar space in diffuse atomic hydrogen gas where the fraction of molecular hydrogen H₂ is in the range of 0.0001 to 0.001.^[1]

Argonium is formed when H₂⁺ reacts with Ar atoms:^[1]

Ar + H⁺
₂ → ArH⁺ + H^[1]

and it is also produced from Ar⁺ ions produced by cosmic rays and X-rays from neutral argon:

Ar⁺ + H₂ → *ArH⁺ + H^[1] 1.49 eV.^[7]

When ArH⁺ encounters an electron, dissociative recombination can occur, but it is extremely slow for lower energy electrons, allowing ArH⁺ to survive for a much longer time than many other similar protonated cations.

ArH⁺ + e⁻ → ArH* → Ar + H^[1]

Artificial ArH⁺ made from earthly Ar contains mostly the isotope ⁴⁰Ar rather than the cosmically abundant ³⁶Ar. Artificially it is made by an electric discharge through an argon-hydrogen mixture.^[8]

Cluster argon cations

The diargon cation, Ar⁺
₂ has a binding energy of 1.29 eV.^[9]

The triargon cation Ar⁺
₃ is linear, but has one Ar−Ar bond shorter than the other. Bond lengths are 2.47 and 2.73 ångströms. The dissociation energy to Ar and Ar₂⁺ is 0.2 eV. In line with the molecule's asymmetry, the charge is calculated as +0.10, +0.58 and +0.32 on each argon atom, so that it greatly resembles Ar⁺
₂ bound to a neutral Ar atom.^[10]

Larger charged argon clusters are also detectable in mass spectroscopy. The tetraargon cation is also linear. Ar⁺
₁₃ icosahedral clusters have an Ar⁺
₃ core, whereas Ar⁺
₁₉ is dioctahedral with an Ar⁺
₄ core. The linear Ar⁺
₄ core has +0.1 charge on the outer atoms, and +0.4 charge on each or the inner atoms. For larger charged argon clusters, the charge is not distributed on more than four atoms. Instead the neutral outer atoms are attracted by induced electric polarization.^[11] The charged argon clusters absorb radiation, from the near infrared, through visible to ultraviolet. The charge core, Ar⁺
₂, Ar⁺
₃ or Ar⁺
₄ is called a chromophore. Its spectrum is modified by the first shell of neutral atoms attached. Larger clusters have the same spectrum as the smaller ones. When photons are absorbed in the chromophore, it is initially electronically excited, but then energy is transferred to the whole cluster in the form of vibration. Excess energy is removed by outer atoms evaporating from the cluster one at a time. The process of destroying a cluster by light is called photofragmentation.^[11]

Negatively-charged argon clusters are thermodynamically unstable, and therefore cannot exist. Argon has a negative electron affinity.^[11]

Argon monohydride

Neutral argon hydride, also known as argon monohydride (ArH), was the first discovered noble gas hydride. J. W. C. Johns discovered an emission line of ArH at 767 nm and announced the find in 1970. The molecule was synthesized using X-ray irradiation of mixtures of argon with hydrogen-rich molecules such as H₂, H₂O, CH₄ and CH₃OH.^[12] The X-ray excited argon atoms are in the 4p state.^[13]

Argon monohydride is unstable in its ground state, 4s, as a neutral inert gas atom and a hydrogen atom repel each other at normal intermolecular distances. When a higher-energy-level ArH* emits a photon and reaches the ground state, the atoms are too close to each other, and they repel and break up. However a van der Waals molecule can exist with a long bond.^[14] However, excited ArH* can form stable Rydberg molecules, also known as excimers. These Rydberg molecules can be considered as a protonated argon core, surrounded by an electron in one of many possible higher energy states.^[15]

Formation: Ar + ν → Ar*;  Ar* + H₂ → ArH* + H^[12]

Instead of dihydrogen, other hydrogen containing molecules can also have a hydrogen atom abstracted by excited argon, but note that some molecules bind hydrogen too strongly for the reaction to proceed. For example, acetylene will not form ArH this way.^[12]

In the van der Waals molecule of ArH, the bond length is calculated to be about 3.6 Å and the dissociation energy calculated to be 0.404 kJ/mol (33.8 cm⁻¹).^[16] The bond length in ArH* is calculated as 1.302 Å.^[17]

The spectrum of argon monohydride, both ArH* and ArD*, has been studied. The lowest bound state is termed A²Σ⁺ or 5s. Another low lying state is known as 4p, made up of C²Σ⁺ and B²π states. Each transition to or from higher level states corresponds to a band. Known bands are 3p → 5s, 4p → 5s, 5p → 5s (band origin 17486.527 cm^−1[18]), 6p → 5s (band origin 21676.90 cm^−1[18]) 3dσ → 4p, 3dπ → 4p (6900 cm⁻¹), 3dδ → 4p (8200–8800 cm⁻¹), 4dσ → 4p (15075 cm⁻¹), 6s → 4p (7400–7950 cm⁻¹), 7s → 4p (predicted at 13970 cm⁻¹, but obscured), 8s → 4p (16750 cm⁻¹), 5dπ → 4p (16460 cm⁻¹), 5p → 6s (band origin 3681.171 cm⁻¹),^[19] 4f → 5s (20682.17 and 20640.90 cm⁻¹ band origin for ArD and ArH), 4f → 3dπ (7548.76 and 7626.58 ccm⁻¹), 4f → 3dδ (6038.47 and 6026.57 cm⁻¹), 4f → 3dσ (4351.44 cm⁻¹ for ArD).^[14] The transitions going to 5s, 3dπ → 5s and 5dπ → 5s, are strongly predissociated, blurring out the lines.^[19] In the UV spectrum a continuous band exists from 200 to 400 nm. This band is due to two different higher states: B²Π → A²Σ⁺ radiates over 210–450 nm, and E²Π → A²Σ⁺ is between 180 and 320 nm.^[20] A band in the near infrared from 760 to 780 nm.^[21]

Other ways to make ArH include a Penning-type discharge tube, or other electric discharges. Yet another way is to create a beam of ArH⁺ (argonium) ions and then neutralize them in laser-energized caesium vapour. By using a beam, the lifetimes of the different energy states can be observed, by measuring the profile of electromagnetic energy emitted at different wavelengths.^[22] The E²π state of ArH has a radiative lifetime of 40 ns. For ArD the lifetime is 61 ns. The B²Π state has a lifetime of 16.6 ns in ArH and 17 ns in ArD.^[20]

Argon polyhydrides

The argon dihydrogen cation ArH⁺
₂ has been predicted to exist and to be detectable in the interstellar medium. However it has not been detected as of 2021.^[23] ArH⁺
₂ is predicted to be linear in the form Ar−H−H. The H−H distance is 0.94 Å. The dissociation barrier is only 2 kcal/mol (8 kJ/mol), and ArH⁺
₂ readily loses a hydrogen atom to yield ArH⁺.^[24] The force constant of the ArH bond in this is 1.895 mdyne/Ų (1.895×10¹² Pa).^[25]

The argon trihydrogen cation ArH⁺
₃ has been observed in the laboratory.^[23][26] ArH₂D⁺, ArHD⁺
₂ and ArD⁺
₃ have also been observed.^[27] The argon trihydrogen cation is planar in shape, with an argon atom off the vertex of a triangle of hydrogen atoms.^[28]

Argoxonium

The argoxonium ion ArOH⁺ is predicted to be bent molecular geometry in the 1¹A′ state. ³Σ⁻ is a triplet state 0.12 eV higher in energy, and ³A″ is a triplet state 0.18 eV higher. The Ar−O bond is predicted to be 1.684 Å long^[23] and to have a force constant of 2.988 mdyne/Ų (2.988×10¹² Pa).^[25]

ArNH⁺

ArNH⁺ is a possible ionic molecule to detect in the lab, and in space, as the atoms that compose it are common. ArNH⁺ is predicted to be more weakly bound than ArOH⁺, with a force constant in the Ar−N bond of 1.866 mdyne/Ų (1.866×10¹² Pa). The angle at the nitrogen atom is predicted to be 97.116°. The Ar−N lengths should be 1.836 Å and the N−H bond length would be 1.046 Å^[25][29]

Argon dinitrogen cation

The argon dinitrogen linear cationic complex has also been detected in the lab:

Ar + N⁺
₂ → ArN⁺
₂ photodissociation→ Ar⁺ + N₂.^[23]

The dissociation yields Ar⁺, as this is a higher-energy state.^[9] The binding energy is 1.19 eV.^[9] The molecule is linear. The distance between two nitrogen atoms is 1.1 Å. This distance is similar to that of neutral N₂ rather than that of N⁺
₂ ion. The distance between one nitrogen and the argon atom is 2.2 Å.^[9] The vibrational band origin for the nitrogen bond in ArN⁺
₂ (V = 0 → 1) is at 2272.2564 cm⁻¹ compared with N₂⁺ at 2175 and N₂ at 2330 cm⁻¹.^[9]

In the process of photodissociation, it is three times more likely to yield Ar⁺ + N₂ compared to Ar + N⁺
₂.^[30]

ArHN⁺
₂

ArHN⁺
₂ has been produced in a supersonic jet expansion of gas and detected by Fourier transform microwave spectroscopy.^[26] The molecule is linear, with the atoms in the order Ar−H−N−N. The Ar−H distance is 1.864 Å. There is a stronger bond between hydrogen and argon than in ArHCO⁺.^[31]

The molecule is made by the following reaction:

ArH⁺ + N₂ → ArHN⁺
₂.^[31]

Bis(dinitrogen) argon cation

The argon ion can bond two molecules of dinitrogen (N₂) to yield an ionic complex with a linear shape and structure N=N−+Ar−N=N. The N=N bond length is 1.1014 Å, and the nitrogen to argon bond length is 2.3602 Å. 1.7 eV of energy is required to break this apart to N₂ and ArN⁺
₂. The band origin of an infrared band due to antisymmetric vibration of the N=N bonds is at 2288.7272 cm⁻¹. Compared to N₂ it is redshifted 41.99 cm⁻¹. The ground state rotational constant of the molecule is 0.034296 cm⁻¹.^[30]

Ar(N
₂)⁺
₂ is produced by a supersonic expansion of a 10:1 mixture of argon with nitrogen through a nozzle, which is impacted by an electron beam.^[30]

ArN₂O⁺

ArN₂O⁺ absorbs photons in four violet–ultraviolet wavelength bands leading to breakup of the molecule. The bands are 445–420, 415–390, 390–370, and 342 nm.^[32][33]

ArHCO⁺

ArHCO⁺ has been produced in a supersonic-jet expansion of gas and detected by Fabry–Perot-type Fourier transform microwave spectroscopy.^[26][34]

The molecule is made by this reaction

ArH⁺ + CO → ArHCO⁺.^[31]

Carbon dioxide–argon ion

ArCO⁺
₂ can be excited to form ArCO⁺
₂* where the positive charge is moved from the carbon dioxide part to the argon. This molecule may occur in the upper atmosphere. Experimentally the molecule is made from a low-pressure argon gas with 0.1% carbon dioxide, irradiated by a 150 V electron beam. Argon is ionized, and can transfer the charge to a carbon dioxide molecule.^[35] The dissociation energy of ArCO⁺
₂ is 0.26 eV.^[35]

ArCO⁺
₂ + CO₂ → Ar + CO
₂·CO⁺
₂ (yields 0.435 eV.)^[35]

van der Waals molecules

Neutral argon atoms bind very weakly to other neutral atoms or molecules to form van der Waals molecules. These can be made by expanding argon under high pressure mixed with the atoms of another element. The expansion happens through a tiny hole into a vacuum, and results in cooling to temperatures a few degrees above absolute zero. At higher temperatures the atoms will be too energetic to stay together by way of the weak London dispersion forces. The atoms that are to combine with argon can be produced by evaporation with a laser or alternatively by an electric discharge. The known molecules include AgAr, Ag₂Ar, NaAr, KAr, MgAr, CaAr, SrAr, ZnAr, CdAr, HgAr, SiAr,^[36] InAr, CAr,^[37] GeAr,^[38] SnAr,^[39] and BAr.^[40] SiAr was made from silicon atoms derived from Si(CH₃)₄.^[41]

In addition to the very weakly bound van der Waals molecules, electronically-excited molecules with the same formula exist. As a formula these can be written ArX*, with the "*" indicating an excited state. The atoms are much more strongly bound with a covalent bond. They can be modeled as an ArX⁺ surrounded by a higher energy shell with one electron. This outer electron can change energy by exchanging photons and so can fluoresce. The widely used argon fluoride laser makes use of the ArF* excimer to produce strong ultraviolet radiation at 192 nm. The argon chloride laser using ArCl* produces even shorter ultraviolet at 175 nm, but is too feeble for application.^[42] The argon chloride in this laser comes from argon and chlorine molecules.^[43]

Aqueous argon

Argon dissolved in water causes the pH to rise to 8.0,^[81] apparently by reducing the number of oxygen atoms available to bind protons.^[82]

With ice, argon forms a clathrate hydrate. Up to 0.6 GPa, the clathrate has a cubic structure. Between 0.7 and 1.1 GPa the clathrate has a tetragonal structure. Between 1.1 and 6.0 GPa the structure is body centered orthorhombic. Over 6.1 GPa, the clathrate converts into solid argon and ice VII.^[83] At atmospheric pressure the clathrate is stable below 147 K.^[84] At 295 K the argon pressure from the clathrate is 108 MPa.^[85]

Argon fluorohydride

Argon fluorohydride was an important discovery in the rejuvenation of the study of noble gas chemistry. HArF is stable in solid form at temperatures below 17 K.^[86] It is prepared by photolysis of hydrogen fluoride in a solid argon matrix.^[87] HArArF would have such a low barrier to decomposition that it will likely never be observed.^[88] However HBeArF is predicted to be more stable than HArF.^[89]

Uranium compounds

CUO in a solid argon matrix can bind one, or a few argon atoms to yield CUO·Ar, CUO·Ar₃ or CUO·Ar₄. CUO itself is made by evaporating uranium atoms into carbon monoxide. Uranium acts as a strong Lewis acid in CUO^[87][90] and forms bonds with energies of about 3.2 kcal/mol (13.4 kJ/mol) with argon. The argon acts as a Lewis base. Its electron density is inserted into an empty 6d orbital on the uranium atom. The spectrum of CUO is changed by argon so that the U−O stretch frequency changes from 872.2 to 804.3 cm⁻¹ and the U−C stretch frequency from 1047.3 to 852.5 cm⁻¹.^[91] The significant change in the spectrum occurs because the CUO is changed from a singlet state (in gas phase or solid neon) to a triplet state, with argon or noble gas complexing.^[92] The argon–uranium bond length is 3.16 Å.^[91] This is shorter than the sum of atomic radii of U and Ar of 3.25 Å, but considerably longer than a normal covalent bond to uranium. For example, U−Cl in UCl₆ is 2.49 Å.^[92] When xenon is included in the solid argon matrix up to a few percent, additional van der Waals molecules are formed: CUO·Ar₃Xe, CUO·Ar₂Xe₂, CUO·ArXe₃ and CUO·Xe₄.^[90] Similarly krypton can substitute for argon in CUO·Ar₃Kr, CUO·Ar₂Kr₂, CUO·ArKr₃ and CUO·Kr₄.^[92] The shape of these molecules is roughly octahedral, with a uranium centre and with the noble gas atoms around the equator.^[92]

UO⁺
₂ can bind up to five noble gas atoms in a ring around a linear O=+U=O core.^[93] These molecules are produced when uranium metal is laser ablated into dioxygen. This produces UO, UO₂, UO₃, U⁺, and importantly UO⁺
₂. UO⁺
₂ is then condensed into a noble gas matrix, either a pure element or a mixture. Heavier noble gas atoms will tend to displace the lighter atoms. Ionic molecules produced this way include UO
₂Ne
₄Ar⁺
, UO
₂Ne
₃Ar⁺
₂, UO
₂Ne
₂Ar⁺
₃, UO
₂NeAr⁺
₄, UO
₂Ar⁺
₅, UO
₂Ar
₄Kr⁺
, UO
₂Ar
₃Kr⁺
₂, UO
₂Ar
₂Kr⁺
₃, UO
₂ArKr⁺
₄, UO
₂Ar
₄Xe⁺
, UO
₂Ar
₃Xe⁺
₂, UO
₂Ar
₂Xe⁺
₃, and UO
₂ArXe⁺
₄, which are identified by a shift in the U=O antisymmetric stretching frequency.^[93]

Neutral UO₂ condensed in solid argon is converted from one electronic state to another by the argon atom ligands. In argon the electron configuration is 5f²(δφ) whereas in neon it is 5f¹7s¹ (the state ³H_4g compared to ³Φ_2u). This is because the argon atoms have a larger antibonding interaction with the 7s¹ electron, forcing it into a different subshell. The argonated compound has a stretching frequency of 776 cm⁻¹ compared to 914.8 cm⁻¹ in neon.^[94] The argon uranium dioxide molecule is likely UO₂Ar₅.^[95]

Beryllium oxide

When beryllium atoms react with oxygen in a solid argon matrix (or beryllia is evaporated into the matrix) ArBeO will be formed, and is observable by its infrared spectrum. The beryllia molecule is strongly polarised, and the argon atom is attracted to the beryllium atom.^[92][96] The bond strength of Ar−Be is calculated to be 6.7 kcal/mol (28 kJ/mol). The Ar−Be bond length is predicted to be 2.042 Å.^[97]

The cyclic Be₂O₂ molecule can bind two argon atoms, or one argon along with another noble gas atom.^[98]

Analogously, beryllium reacting with hydrogen sulfide and trapped in an argon matrix at 4 K forms ArBeS. It has a binding energy calculated to be 12.8 kcal/mol (54 kJ/mol).^[99]

ArBeO₂CO (beryllium carbonate) has been prepared (along with Ne, Kr and Xe adducts).^[100]

The cyclic beryllium sulfite molecule can also coordinate an argon atom onto the beryllium atom in solid neon or argon matrix.^[101]

Carbonyl compounds

Group 6 elements can form reactive pentacarbonyls that can react with argon. These were actually argon compounds discovered in 1975, and were known before the discovery of HArF, but are usually overlooked.^[102] Tungsten normally forms a hexacarbonyl, but when subject to ultraviolet radiation it breaks into a reactive pentacarbonyl. When this is condensed into a noble gas matrix the infrared and UV spectrum varies considerably depending on the noble gas used. This is because the noble gas present binds to the vacant position on the tungsten atom. Similar results also occur with molybdenum and chromium.^[103] Argon is only very weakly bound to tungsten in ArW(CO)₅.^[92][104] The Ar−W bondlength is predicted to be 2.852 Å.^[103] The same substance is produced for a brief time in supercritical argon at 21 °C.^[105] For ArCr(CO)₅ the band maximum is at 533 nm (compared to 624 nm in neon, and 518 nm in krypton). Forming 18-electron complexes, the shift in spectrum due to different matrices was much smaller, only around 5 nm. This clearly indicates the formation of a molecule using atoms from the matrix.^[5]

Other carbonyls and complexed carbonyls also have reports of bonding to argon. These include Ru(CO)₂(PMe₃)₂Ar, Ru(CO)₂(dmpe)₂Ar, η⁶-C₆H₆Cr(CO)₂Ar.^[106] Evidence also exists for ArHMn(CO)₄, ArCH₃Mn(CO)₄, and fac-(η²-dfepe)Cr(CO)₃Ar.^[5]

Other noble gas complexes have been studied by photolysis of carbonyls dissolved in liquid rare gas, possibly under pressure. These Kr or Xe complexes decay on the time scale of seconds, but argon does not seem to have been studied this way. The advantage of liquid noble gases is that the medium is completely transparent to infrared radiation, which is needed to study the bond vibration in the solute.^[5]

Attempts have been made to study carbonyl–argon adducts in the gas phase, but the interaction appears to be too weak to observe a spectrum. In the gas form, the absorption lines are broadened into bands because of rotation that happens freely in a gas.^[5] The argon adducts in liquids or gases are unstable as the molecules easily react with the other photolysis products, or dimerize, eliminating argon.^[5]

Coinage metal monohalides

The argon coinage metal monohalides were the first noble gas metal halides discovered, when the metal monohalide molecules were put through an argon jet. There were first found in Vancouver in 2000.^[107] ArMX with M = Cu, Ag or Au and X = F,^[108] Cl or Br have been prepared. The molecules are linear. In ArAuCl the Ar−Au bond is 2.47 Å, the stretching frequency is 198 cm⁻¹ and the dissociation energy is 47 kJ/mol.^[109] ArAgBr also has been made.^[109] ArAgF has a dissociation energy of 21 kJ/mol.^[109] The Ar−Ag bond-length in these molecules is 2.6 Å.^[109] ArAgCl is isoelectronic with AgCl⁻
₂ which is better known.^[109] The Ar−Cu bond length in these molecules is 2.25 Å.^[109]

Transition metal oxides

In a solid argon matrix VO₂ forms VO₂Ar₂, and VO₄ forms VO₄·Ar with binding energy calculated to be 12.8 and 5.0 kcal/mol (53 and 21 kJ/mol).^[110] Scandium in the form of ScO⁺ coordinates five argon atoms to yield ScOAr⁺
₅.^[111] these argon atoms can be substituted by numbers of krypton or xenon atoms to yield even more mixed noble gas molecules. With yttrium, YO⁺ bonds six argon atoms, and these too can be substituted by varying numbers of krypton or xenon atoms.^[112]

In the case of transition metal monoxides, ScO, TiO and VO do not form a molecule with one argon atom. However CrO, MnO, FeO, CoO and NiO can each coordinate one argon atom in a solid argon matrix.^[113] The metal monoxide molecules can be produced by laser ablation of the metal trioxide, followed by condensation on solid argon. ArCrO absorbs at 846.3 cm⁻¹, ArMnO at 833.1, ArFeO at 872.8, ArCoO at 846.2, Ar⁵⁸NiO at 825.7 and Ar⁶⁰NiO at 822.8 cm⁻¹. All these molecules are linear.^[113]

There are also claims of argon forming coordination molecules in NbO₂Ar₂, NbO₄Ar, TaO₄Ar,^[114] VO₂Ar₂, VO₄Ar,^[110] Rh(η²-O₂)Ar₂, Rh(η²-O₂)₂Ar₂, Rh(η²-O₂)₂(η¹-OO)Ar.^{[115][116][117]}

Tungsten trioxide, WO₃, and tungsten dioxide mono-superoxide (η²-O₂)WO₂ can both coordinate argon in an argon matrix. The argon can be replaced by xenon or molecular oxygen to make xenon coordinated compounds or superoxides. For WO₃Ar the binding energy is 9.4 kcal/mol and for (η²-O₂)WO₂ it is 8.1 kcal/mol.^[118]

Other transition metal compounds

ArNiN₂ binds argon with 11.52 kcal/mol. The bending frequency of ArN₂ is changed from 310.7 to 358.7 cm⁻¹ when argon attaches to the nickel atom.^[119]

Other ions

Some other binary ions observed that contain argon include BaAr²⁺ and BaAr²⁺
₂,^[120] VAr⁺, CrAr⁺, FeAr⁺, CoAr⁺, and NiAr⁺.^[5]

Gold and silver cluster ions can bind argon. Known ions are Au
₃Ar⁺
, Au
₃Ar⁺
₂, Au
₃Ar⁺
₃, Au
₂AgAr⁺
₃ and AuAg
₂Ar⁺
₃. These have a triangular shaped metallic core with argon bound at the vertexes.^[2]

ArF⁺ is also known^[5] to be formed in the reaction

F⁺
₂ + Ar → ArF⁺ + F

and also

Ar⁺ + F₂ → ArF⁺ + F.

and also

SF²⁺
₄ + Ar → ArF⁺ + SF⁺
₃.^[121]

The ions can be produced by ultraviolet light at 79.1 nm or less.^[122] The ionisation energy of fluorine is higher than that of argon, so breakup occurs thus:

ArF⁺ → Ar⁺ + F.^[123]

The millimeter wave spectrum of ArF⁺ between 119.0232 and 505.3155 GHz has been measured to calculate molecular constants B₀ = 14.8788204 GHz, D₀ = 28.718 kHz.^[124] There is a possibility that a solid salt of ArF⁺ could be prepared with SbF⁻
₆ or AuF⁻
₆ anions.^[123][125]

Excited or ionized argon atoms can react with molecular iodine gas to yield ArI^+[126] Argon plasma is used as an ionisation source and carrier gas in inductively coupled plasma mass spectrometry. This plasma reacts with samples to produce monatomic ions, but also forms argon oxide (ArO⁺), and argon nitride (ArN⁺) cations, which can cause isobaric interference with detection and measurement of iron-56 (⁵⁶Fe) and iron-54 (⁵⁴Fe), respectively, in mass spectrometry.^[127] Platinum present in stainless steel can form platinum argide (PtAr⁺) which interferes with the detection of uranium-234 which can be used as a tracer in aquifers.^[128] Argon chloride cations can interfere with the detection of arsenic as Ar³⁵Cl⁺ has a mass-to-charge ratio almost identical to that of arsenic's one stable isotope, ⁷⁵As.^[129] In these circumstances ArO⁺ may be removed by reaction with NH₃.^[130] Alternatively electrothermal vaporization or using helium gas can avoid these interference problems.^[127] Argon can also form an anion with chlorine, ArCl⁻,^[131] though this is not a problem for mass spectrometry applications as only cations are detected.

The argon borynium ion, BAr⁺ is produced when BBr⁺ at energies between 9 and 11 eV reacts with argon atoms. 90% of the positive charge is on the argon atom.^[132]

ArC⁺ ions can be formed when argon ions impact carbon monoxide with energies between 21 and 60 eV. However more C⁺ ions are formed, and when the energy is on the high side, O⁺ is higher.^[133]

ArN⁺ can form when argon ions impact dinitrogen with energies between 8.2 and 41.2 eV and peaking around 35 eV. However far more N⁺
₂ and N⁺ are produced.^[134]

ArXe⁺ is held together with a strength of 1445 cm⁻¹ when it is in the X electronic state, but 1013 cm⁻¹ when it is in the B excited state.^[33]

Metal–argon cations are called "argides". The argide ions produced during mass spectroscopy have higher intensity when the binding energy of the ion is higher. Transition elements have higher binding and ion flux intensity compared to main group elements. Argides can be formed in the plasma by excited argon atoms reacting with another element atom, or by an argon atom binding with another ion:

Ar⁺ + M → ArM⁺ + e⁻; M⁺ + Ar → ArM⁺.^[135]

Doubly charged cations, called superelectrophiles, are capable of reacting with argon. Ions produced include ArCF²⁺
₂ ArCH⁺
₂, ArBF⁺
₂ and ArBF²⁺
containing bonds between argon and carbon or boron.^[136]

Doubly ionised acetylene HCCH²⁺ reacts inefficiently with argon to yield HCCAr²⁺. This product competes with the formation of Ar⁺ and argonium.^[137]

The SiF²⁺
₃ ion reacts with argon to yield ArSiF²⁺
₂.^[138]

Ion	Bond length (Å)	Dissociation energy (kJ/mol)[5]	Excited state bond length (Å)	Excited state dissociation energy
ArH+		3.4 eV
LiAr+[135]	2.343	0.30 eV
BeAr+[135]		4100 cm−1[139]
BAr+[132]	2.590	210
ArC+[140]
ArN+[135]	3.5	2.16 eV[141]
ArO+[135]
ArF+[123]	1.637	194
NaAr+[135]		19.3
MgAr+[135]	2.88[142]	1200 cm−1[139]
AlAr+[135]		982 cm−1[143]
SiAr+[135]
ArP+[135]
ArS+[135]
ArCl+[135]
Ar+ 2[135]
CaAr+		700 cm−1[139]
ScAr+[135]
TiAr+		0.31eV[144]
VAr+	2.65[145]	37,D0=2974 cm−1[142]
CrAr+		28,D0=2340[142]
MnAr+[135]		0.149 eV[144]
FeAr+		0.11 eV[144]
CoAr+[145]	2.385[146]	49,D0=4111 cm−1[146]
NiAr+		53,D0=4572[142]
CuAr+[135]		0.53 eV[144]
ZnAr+	2.72[147]	0.25 eV,[144] D0=2706 cm−1[147]
GaAr+[135]
AsAr+[135]
RbAr+[148]
SrAr+		800[139]
ZrAr+	2.72	D0 = 2706 cm−1[147]	3.050	1179 cm−1
NbAr+	2.677[142]	37,D0=3106 cm−1[142]
AgAr+[135]
InAr+[149]
ArI+[126]
BaAr+		600 cm−1[139]

Polyatomic anions

Ball-and-stick model of the complex of superelectrophilic anion [B₁₂(CN)₁₁]⁻ with Ar. B₁₂ core has nearly icosahedral symmetry. B – pink, C – grey, N – dark blue, Ar – blue.

Examples of anions containing strong bonds with noble gases are extremely rare: generally nucleophilic nature of anions results in their inability to bind to noble gases with their negative electron affinity. However, the 2017 discovery of "superelectrophilic anions",^[166] gas phase fragmentation products of closo-dodecaborates, led to the observation of stable anionic compounds containing a boron-noble gas bond with significant degree of covalent interaction. The most reactive superelectrophilic anion [B₁₂(CN)₁₁]⁻, fragmentation product of cyanated cluster [B₁₂(CN)₁₂]^2-, was reported to bind argon spontaneously at room temperature.^[4]

Solid compounds

Armand Gautier noticed that rock contained argon (and also nitrogen) that was liberated when the rock was dissolved in acid^[167] however how the argon was combined in rock was ignored by the scientific community.^[168]

Organoargon chemistry

Organoargon chemistry describes the synthesis and properties of chemical compounds containing a carbon to argon chemical bond.

Very few such compounds are known. The reaction of acetylene dications with argon produced HCCAr²⁺ in 2008.^[183] Reaction of the CF2+3 dication with argon produced ArCF2+2: this reaction is unique to argon among the noble gases.^[184]

The compound FArCCH has been theoretically studied and is predicted to be stable.^[185] FArCCF might also be stable enough to synthesise and detect, but probably not FArCCArF.^[186] Calculations in 2015 suggest that FArCCH and FArCH₃ are stable, but not FArCN.^[187] FArCC⁻ should be kinetically stable, as is also expected of the krypton and xenon (but not helium) analogues.^[188] HArC₄H (for which the krypton analogue is known) and HArC₆H have also been predicted as stable.^[189] FArCO⁺ and ClArCO⁺ should be metastable and might be possible to characterise under cryogenic conditions.^[190] Calculations suggest that HArCCF and HCCArF should be stable, and that HNgCCF molecules should be more stable than HNgCCH (Ng = Ar, Kr, Xe); the corresponding krypton species have been experimentally produced, but not the argon species despite an experimental attempt. HCCNgCN and HCCNgNC (Ng = Ar, Kr, Xe) are likewise computed to be stable, but experimental searches for them have failed.^[191]

References

External links

• Linnartz, H.; Verdes, D.; Maier, J. P. (16 August 2002). "Rotationally Resolved Infrared Spectrum of the Charge Transfer Complex [Ar–N₂]⁺ Supporting Online Material Observed frequencies and observed-calculated values of rotational transitions in the ²Σ⁺ ground state of [ArN₂]⁺ upon excitation of the NN stretching fundamental". Science. 297 (5584): 1166–1167. Bibcode:2002Sci...297.1166L. doi:10.1126/science.1074201. PMID 12183626. S2CID 45850131.
• Nonose, Naoko S.; Matsuda, Naoki; Fudagawa, Noriko; Kubota, Masaaki (1994). "Some characteristics of polyatomic ion spectra in inductively coupled plasma mass spectrometry". Spectrochimica Acta Part B: Atomic Spectroscopy. 49 (10): 955. Bibcode:1994AcSpe..49..955N. doi:10.1016/0584-8547(94)80084-7.
• Jiménez Redondo, Miguel; Cueto, Maite; Doménech, José Luis; Tanarro, Isabel; Herrero, Víctor J. (2014). "Ion kinetics in Ar/H₂ cold plasmas: The relevance of ArH⁺". RSC Adv. 4 (107): 62030–62041. Bibcode:2014RSCAd...462030J. doi:10.1039/C4RA13102A. PMC 4685740. PMID 26702354.