Helium compounds

Helium is the most unreactive element and so it is commonly believed that helium compounds do not exist. 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 or 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. The atom is very hard with Pearson's 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.

Repulsive forces between helium and other atoms may be overcome by high pressures. Helium has been shown to form a crystalline compound with sodium under pressure. Suitable pressures to force helium into solid combinations could be found inside planets. Clathrates are also possible with helium under pressure in ice, and other small molecules such as nitrogen.

Other ways to make helium reactive, are to convert it into an ion, or to excite an electron to a higher level, allowing it to form excimers. Ionised helium, also known as He II, is a very high energy material able to extract an electron from any other atom. Excimers do not last for long, as the molecule containing the higher energy level helium atom can rapidly decay back to a repulsive ground state, where the two atoms making up the bond repel. However in some locations such as helium white dwarfs, conditions may be suitable to rapidly form excited helium atoms. The excited helium atom has a 1S electron promoted to 2S. This requires 460 kcal/gram of energy, which can be supplied by electron impact, or electric discharge.

Known high pressure phases

Disodium helide

HeNa₂ was first predicted using USPEX code. It was predicted to be thermodynamically stable over 160 GPa and dynamically stable over 100 GPa. This means it should be possible to form at the higher pressure and then decompress to 100 GPa, but below that it would decompose. Compared with other binary compounds of other elements and helium, it was predicted to be stable at the lowest pressures. HeNa₂ has a cubic crystal structure, resembling fluorite. At 300 GPa a=3.95 Å. Each unit cell contains four helium atoms on the centre of the cube faces and corners, and eight sodium atoms at coordinates a quarter cell in from each face. Double electrons are positioned on each edge and the centre of the cell. Each pair of electrons is spin paired. The presence of these isolated electgrons makes this an electride. The material was made in a diamond anvil cell at 130 GPa heated to 1500K with a laser. known HeNa₂ is predicted to be an insulator and transparent. The sodium atoms have an average charge of +0.6, the helium is -0.15 and the 2e spots are -1.1 (so not really two whole electrons).

Titanate

• La_2/3-xLi_3xTiO₃He (like a clathrate)

Silicates

Crystobalite He II (SiO₂He) is stable between 1.7 and 6.4 Gpa. It has a rhombohedral space group R-3c with unit cell dimensions a=9.080 α=31.809° V=184.77 Å₃ at 4GPa.

Crystobalite He I (SiO₂He) is formed under higher helium pressures over 6.4 Gpa. It has a monoclinic space group P2₁/C with unit cell dimensions a=8.062 b=4.797 c=9.491 Å β=120.43° V=316.47 Å₃ at 10 Gpa.

Helium penetrates into silica glass with high pressure yielding a silica glass helium and reduces its compressibility.

Dihelium arsenolite

Dihelium arsenolite As₄O₆•2He is stable from pressures over 5 GPa and up to at least 30 GPa. Arsenolite is one of the softest and most compressible minerals.

Small molecule

• He(N₂)₁₁ a van der Waals compound with hexagonal crystals. At 10 GPa the unit cell of 22 nitrogen atoms has a unit cell volume of 558 Å³, and about 512 Å³ at 15 GPa. These sizes are around 10 Å³ smaller than the equivalent amount of solid δ-N₂ nitrogen at these pressures.
• NeHe₂ hexagonal MgZn₂ type; at 13.7 GPa a=4.066 Å c=6.616 Å; at 21.8 GPa a=3.885 Å c=6.328 Å; Z=4; melts at 12.8 Gpa and 296 K, stable to over 90 Gpa.

Clathrates

Helium clathrates only form under pressure. With ice II at pressures between 280 and 480 MPa a solid helium hydrate with He:H₂O ratio of 1:6 exists. Other helium hydrates with the ice-I_h, ice-I_c 1:1, and ice-I_c 2:1 He to H₂O ratio have been predicted. Helium clathrate hydrates should be similar to hydrogen clathrate due to the similar size of the hydrogen molecule.

Impurity helium condensates

Impurity helium condensates (IHCs) (or impurity helium gels) are deposited as a snow like gel in liquid helium when various atoms or molecules are absorbed on the surface of superfluid helium. Atoms can include H, N, Na, Ne, Ar, Kr, Xe, alkalis or alkaline earths. The impurities form nanoparticle clusters coated with localised helium held by van der Waals force. Helium atoms are unable to move towards or away from the impurity, but perhaps can move perpendicularly around the impurity. The snow like solid is structured like an aerogel. When free atoms are included in the condensate a high energy density can be achieved, up to 860 Jcm⁻¹ or 5 kJg⁻¹. These condensates were first investigated as a possible rocket fuel. The mixtures are given a notation involving square brackets so that [N]/[He] represents a nitrogen atom impurity in helium.

[N]/[He] atomic nitrogen impurity helium is produced when a radio frequency discharge in a nitrogen helium mixture is absorbed into superfluid helium, it can have up to 4% nitrogen atoms included. The substance resembles crumbly snow and condenses and settles from the liquid helium. It also contains variable proportions of N₂ molecules. This substance is a high energy solid, with as much power as convention explosives. When it is heated above 2.19K (the lambda point of helium) the solid decomposes and explodes. This substance is not a true compound, but more like a solid solution. E. B. Gordon et al suggested that this material may exist in 1974. The localised helium shells around an individual atom are termed van der Waals spheres. However the idea that the nitrogen atoms are dispersed in the helium has been replaced by concept of nitrogen atoms attached to the surface of clusters of nitrogen molecules. The energy density of the solid can be increased by pressing it.

Other inert gas impurity helium condensates can also be made from a gas beam into superfluid He. [Ne]/[He] decomposes at 8.5 K with release of heat and formation of solid neon. Its composition approximates NeHe₁₆.

[Ar]/[He] contains 40-60 He per Ar

[Kr]/[He] contains 40-60 He per Kr and is stable up to 20 K.

[Xe]/[He] contains 40-60 He per Xe

[N₂]/[He] contains 12-17 He per N₂ molecule. It is stable up to 13 K

[N]/[Ne]/[He] Formed from gas beam generated from RF discharge in mixtures of neon, nitrogen and helium into superfluid He. Additional inert gas stablises more nitrogen atoms. It decomposes around 7 K with a blue green light flash. Excited nitrogen atoms in the N(²D) state can be relative long lasting, up to hours, and give off a green luminescence.

[H₂]/[He], or [D₂]/[He] when dihydrogen or dideuterium is absorbed into superfluid helium, filaments are formed. When enough of these form the solid resembles cotton, rather than snow. Using H₂ results in the product floating and stopping further production, but with deuterium, or a half-half mixture, it can sink and accumulate. Atomic hydrogen in impurity helium decays fairly rapidly due to quantum tunneling (H + H → H₂). Atomic deuterium dimerises slower(D + D → D₂), but reacts very quickly with any diprotium present. (D + H₂ → HD + H). Atomic hydrogen solids are further stabilised by other noble gases such as krypton. Lowering temperatures into the milikelvin range can prolong the lifetime of atomic hydrogen condensates. Condensates containing heavy water or deuterium are under investigation for the production of ultracold neutrons.

The water-helium condensate [H₂O]/[He] contains water clusters of several nanometers in diameter, and pores from 8 to 800 nm.

Oxygen O₂ impurity helium contains solid oxygen clusters from 1 to 100 nm.

Impurity solid helium

Introducing impurities into solid helium yields a blue coloured solid that melts at a higher temperature than pure He. For cesium the absorption has a peak at 750 nm, and for rubidium, maximal absorption is at 640 nm. These are due to metal clusters with diameters of 10 nm or so. However the low concentration of clusters in this substance should not be sufficient to solidify helium as the amount of metal in the solid is less than billionth that of the impurity helium condensate solids, and liquid helium does not "wet" cesium metal. The solid is possibly due to helium snowballs attached to Cs⁺ (or Rb⁺) ions. Free electrons in liquid helium are enclosed in a bubble 17 Å in diameter. Under 25 atmosphere pressure an electron bubble reduces to 11 Å.

Solid solution

Helium can dissolve to a limited extent in hot metal, with concentration proportional to pressure. At atmospheric pressure, 500°C bismuth can absorb 1 part in a billion; at 649°C lithium can take 5 parts per billion; and at 482°C potassium can take 2.9 parts per million (all atom fractions). In nickel there can be 1 in 10¹⁰ atoms, and in gold 1 in 10⁷. The supposition is that the higher the melting point the less helium can be dissolved. However when a liquid metal is quenched, higher concentrations of helium can be left dissolved. So cooled liquid steel can have one part in a million of helium. In order to get a helium atom into a metal lattice, a hole has to be formed. The energy to make that hole in the metal is basically the heat of solution.

Nanowires

Gold, copper, rubidium, cesium or barium atoms evaporated into liquid helium form spider web like structures. Rhenium produces nano flakes. Molybdenum, tungsten, and niobium produce thin nano wires with diameters of 20, 25 and 40 Å. Indium, tin, lead and nickel produce nano wires about 80 Å in diameter. These same four metals also produce smooth spheres about 2 μm across that explode when examined with an electron microscope. Copper, permalloy and bismuth also make nanowires.

Two dimensional ionic crystal

Helium II ions (He⁺) in liquid helium when attracted by an electric field can form a two dimensional crystal at temperatures below 100 mK. There are about half a trillion ions per square meter just below the surface of the helium. Free electrons float above the helium surface.

Known van der Waals molecules

• LiHe
• dihelium
• trihelium
• Ag³He
• HeCO is weakly bound by van der Waals forces. It is potentially important in cold interstellar media as both CO and He are common.

CF₄He and CCl₄He both exist.

Known ions

Helium has the highest ionisation energy, so a He⁺ ion will strip electrons off any other neutral atom or molecule. However it can also then bind to the ion produced. The He⁺ ion can be studied in gas, or in liquid helium. Its chemistry is not completely boring. For example He⁺ can react with SF₆ to yield SF₆⁺ or SF₅⁺ and atomic fluorine.

Ionised clusters

He₂⁺ was predicted to exist by Linus Pauling in 1933. It was discovered when doing mass spectroscopy on ionised helium. The dihelium cation is formed by an ionised helium atom combining with a helium atom: He⁺ + He → He₂⁺.

The diionised dihelium He₂²⁺ (¹Σ_g⁺) is in a singlet state. It breaks up He₂²⁺ → He⁺ + He⁺ releasing 200 kcal/mol of energy. It has a barrier to decomposition of 35 kcal/mol and a bond length of 0.70 Å.

The trihelium cation He₃⁺ is in equilibrium with He₂⁺ between 135 and 200K

Helium hydride

The helium hydride ion HeH⁺ has been known since 1925. The protonated dihelium ion He₂H⁺ can be formed when the dihelium cation reacts with dihydrogen: He₂⁺ + H₂ → He₂H⁺ + H. This is believed to be a linear molecule. Larger protonated helium cluster ions exist He_nH⁺ with n from 3 to 14. He₆H⁺ and He₁₃H⁺ appear to be more common. These can be made by reacting the H₂⁺ or the H₃⁺ with gaseous helium.

HeH²⁺ is unstable in its ground state. But when it is excited to the 2pσ state the molecule is bound with an energy of 20 lcalmol⁻¹. This doubly charged ion has been made by accelerating the helium hydride ion to 900 keV, and firing it in to Argon. It only has a short life of 4 ns.

H₂He⁺ has been made and could occur in nature via H₂ + He⁺ → H₂He⁺.

H₃He_n⁺ exists for N from 1 to over 30, and there are also clusters with more hydrogen atoms and helium.

Noble gas

Noble gas cluster ions exist for different noble gases. Singly charged cluster ions containing xenon exist with the formula He_nXe_m⁺, where n and m ≥ 1.

Many different He_nKr⁺ exist with n=1 to 17 at least. He_nKr₂⁺ and He_nKr₃⁺ also exist for many values of n. He₁₂Kr₂⁺ and He₁₂Kr₃⁺ ions are commons. These singly charged cluster ions can be made from krypton in helium nanodroplets subject to vacuum ultraviolet radiation.

The Ar⁺ Argon ion can form many different sized clusters with helium ranging from HeAr⁺ to He₅₀Ar⁺, but the most common clusters are He₁₂Ar⁺ and smaller. These clusters are made by capturing an argon atom in a liquid helium nanodroplet, and then ionising with high speed electrons. He⁺ is formed, which can transfer charge to argon and then form a cluster ion when the rest of the droplet evaporates. NeHe_n⁺ can be made by ultraviolet photoionisation. Clusters only contain one neon atom. The number of helium atoms n can vary from 1 to 23, but NeHe₄⁺ and NeHe₈⁺ are more likely to be observed.

Doubly charged ions of helium with noble gas atoms also exist including ArHe²⁺, KrHe²⁺, and XeHe²⁺.

Metals

There are various known metal-helium ions.

Alkali metal heliide ions are known for all the alkalis. The molecule ground state for the diatomic ions is in the X¹Σ⁺ state. The bond length gets bigger as the periodic table is descended with lengths of 1.96, 2.41, 2.90, 3.10, and 3.38 Å for Li⁺He, Na⁺He, K⁺He, Rb⁺He, and Cs⁺He. The dissociation energies are 1.9, 0.9, 0.5, 0.4 and 0.3 kcal/mol, showing bond energy decreases. When the molecule breaks up the positive charge is never on the helium atom.

When there are many helium atoms around alkali metal ions can attract helium atoms in shells. Clusters can be formed from absorbing metal into helium droplets. The doped droplets are ionised with high speed electrons. For sodium clusters appear with the formula Na⁺He_n with n from 1 to 26. Na⁺He is the most common, but Na⁺He₂ is very close in abundance. Na⁺He₈ is much more abundant than clusters with more helium. Na₂⁺He_n with n from 1 to 20 also appears. Na₃⁺He_n with small n is also made. For potassium K⁺He_n with n up to 28, and K₂⁺He_n for n from 1 to 20 is formed. K⁺He and K⁺He₂ are both common, and K⁺He₁₂ is a bit more commonly formed than other similar sized clusters. Cesium and rubidium cations also form clusters with helium.

Other known metal helium ions include Cr⁺He, Co⁺He, Co⁺He₃, Ni⁺He, and Ni⁺He₃. PtHe²⁺; formed by high electric field off platinum surface in helium, VHe²⁺, HeRh²⁺ is decomposed in high strength electric field, Ta²⁺He, Mo²⁺He, W²⁺He, Re²⁺He, Ir²⁺He, Pt²⁺He₂, W³⁺He₂, W³⁺He₃ and W³⁺He₄.

Nonmetals

HeN₂⁺ can form around 4K from an ion beam of N₂⁺ into cold helium gas. He_nN₂⁺ with n from 2 to 6 have been made by shooting electrons at a supersonicly expanding mix of nitrogen and helium.

C₆₀He⁺ is formed by irradiating C₆₀ with 50eV electrons and then steering ions into cold helium gas. C₆₀He₂⁺ is also known.

He(OH)⁺ has been detected, although it is not produced when HOT (tritiated water) decays.

He_n(CO)⁺ has been detected for values of n from 1 to 12. Also CH₃He⁺, OCHHe⁺ and NH₂He⁺ have been detected.

Young and Coggiola claimed to make HeC⁺ by an electric discharge off graphite into helium.

When tritium substituted methane (CH₃T) decays CH₃He⁺ is produced in a very small amount.

Excimers

The He₂^* excimer is responsible for the Hopfield continuum. Helium also forms an excimer with barium, Ba⁺He^*.

Endohedral

Helium atoms can be trapped inside molecular cages such as the fullerenes He@C₆₀, He@C₇₀, He₂@C₆₀ and He₂@C₇₀ have all been made using compressed helium and fullerenes.

Other cage like inorganic or organic molecules may also trap helium, for example C₈He with He inside a cube., or He@Mo₆Cl₈F₆.

Predicted solids

He(H₂O)₂ is predicted to form a solid with orthorhomic structure Ibam.

FeHe iron helide was early on claimed to have been found, but the discovery is unlikely. However it is predicted to exist as an interstitial compound under high pressure. It perhaps can exist in dense planetary cores.

Na₂HeO is predicted to have a similar structure to Na₂He, but with oxygen atoms in the same position as the electron pair, so that it becomes O²⁻. It would be stable from 13 to 106 GPa.

Predicted van der Waals molecules

The beryllium oxide helium adduct, HeBeO is believe to be bonded much more strongly than a normal van der Waals molecule with about 5 kcal/mol of binding energy. The bond is enhanced by a dipole induced positive charge on beryllium, and a vacancy in the σ orbital on beryllium where it faces the helium.

Variations on the beryllium oxide adduct include HeBe₂O₂, RNBeHe including HNBeHe, CH₃NBeHe, CH_4-xNBeHe_x, SiH_4-xNBeHe_x, NH_3-xNBeHe_x, PH_3-xNBeHe_x, OH_2-xNBeHe_x, SH_2-xNBeHe_x

Hydridohelium fluoride HHeF is predicted to have a lifetime 157 femto seconds 05 kcal/mol barrier. The life time of the deuterium isotopomer is predicted to be much longer due to a greater difficulty of tunneling for deuterium. This molecule's metastability is slated due to electrostatic attraction between HHe⁺ and F⁻ which increases the barrier to an exothermic breakup. Under pressures over 23 Gpa HHeF should be stable.

Calculations for coinage metal fluorides include HeCuF as stable, HeAgF is unstable, HeAuF is predicted, and Ag³He with binding energy 1.4 cm⁻¹, Ag⁴He binding energy 1.85 cm⁻¹, Au³He binding energy 4.91 cm⁻¹, and Au⁴He binding energy 5.87 cm⁻¹

• HeNaO predicted

Calculation for binary van der Waals helium molecules include HeNe, Li⁴He binding energy 0.008 cm⁻¹, the ³He is not stable. Na⁴He binding energy 0.03 cm⁻¹, the ³He is not stable. Cu³He binding energy 0.90 cm⁻¹, O⁴He binding energy 5.83 cm⁻¹, S⁴He binding energy 6.34 cm⁻¹, Se⁴He binding energy 6.50 cm⁻¹, F⁴He binding energy 3.85 cm⁻¹, Cl⁴He binding energy 7.48 cm⁻¹, Br⁴He binding energy 7.75 cm⁻¹, I⁴He binding energy 8.40 cm⁻¹, N⁴He binding energy 2.85 cm⁻¹, P⁴He binding energy 3.42 cm⁻¹, As⁴He binding energy 3.49 cm⁻¹, Bi⁴He binding energy 33.26 cm⁻¹, Si⁴He binding energy 1.95 cm⁻¹, Ge⁴He binding energy 2.08 cm⁻¹, CaH⁴He binding energy 0.96 cm⁻¹, NH⁴He binding energy 4.42 cm⁻¹, MnH⁴He binding energy 1.01 cm⁻¹, YbF⁴He binding energy 5.57 cm⁻¹ I₂⁴He or I₂³He,

Bonds are predicted to form to nickel with helium as a weak ligand in HeNiCO and HeNiN₂.

Predicted ions

Fluoroheliate ion

Many ions have been investigated theoretically to see if they could exist. Just about every diatomic cation with helium has been studied. For the diatomic dications, for stability the second ionisation level of the partner atom has to be below the first ionisation level of helium, 24.6 eV. For Li, F, and Ne the ground state is repulsive, so molecules will not form. For N and O the molecule would break up to release He⁺. However HeBe²⁺, HeB²⁺ and HeC²⁺ are predicted to be stable. Also second row elements from Na to Cl are predicted to have a stable HeX²⁺ ion.

HeY³⁺ is predicted to be the lightest stable diatomic triply charged ion. Other possibly thermochemically stable ions include HeZr³⁺, HeHf³⁺, HeLa³⁺, HeNd³⁺, HeCe³⁺, HePr³⁺, HePm³⁺, HeSm³⁺, HeGa³⁺, HeTb³⁺, HeDy^{3+, HeHo3+}, HeEr³⁺, HeTm³⁺, HeLu³⁺ where the third ionisation point is below that of helium.

The Positronium Helide ion PsHe⁺ should be formed when positrons encounter helium.

The Fluoroheliate FHeO^- ion should be stable but salts like LiFHeO are not stable. Media related to at Wikimedia Commons

• HHeCO⁺ theoretical
• FHeS- predicted stable see doi=
• FHeBN⁻
• HRgN²⁺ unlikely
• (HNg⁺)(OH2)(HNg⁺)(OH2) probably unstable

The lithium hydrohelide cation HLiHe⁺ is linear in theory. this molecular ion could exist with big bang nucleosynthsis elements Other hydrohelide cations that exist in theory are HNaHe⁺ sodium hydrohelide cation, HKHe⁺ potassium hydrohelide cation, *HBeHe²⁺ berylium hydrohelide cation, HMgHe²⁺ magnesium hydrohelide cation, and HCaHe²⁺ calcium hydrohelide cation.

HeBeO⁺ is predicedted to have a relatively high binding energy of 25 kcal mol⁻¹.

• HCHe⁺
• HCHeHe⁺

For negative ions the adduct is very weakly bound. Those studied include HeCl^-, HeBr^-, HeF^-, HeO^- and HeS^-.

• FHeS^-
• FHeSe^-
• C₇H₆He²⁺
• C₇H₆HeHe²⁺
• FHeCC⁻
• HHeOH₂⁺
• HHeBF⁺
• HeNC⁺
• HeNN⁺
• HHeNN⁺ H-He 0.765 Å He-N bond length 2.077 Å. Decomposition barrier of 2.3 kJ/mol.

HHeNH₃⁺ is predicted to have a C_2v symmetry and a H-He bond length of 0.768 Å and He-N 1.830. The energy barrier against decompostion to ammonium is 19.1 kJ/mol with an energy release of 563.4 kJ/mol. Decompostion to hydrohelium ion and ammonium releases 126.2 kJ/mol

Discredited or unlikely observations

In 1895 L. Troost and L. Ouvrard believed they had witnessed a reaction between magnesium vapour and helium (and also argon) due to the spectrum of helium disappearing from the tube they were passing it through. In 1906, W. Ternant Cooke claimed to have noticed a reaction of helium with cadmium or mercury vapour by observing and increase in the density of the vapour. Zinc vapour did not react with helium.

J. J. Manley claimed to have found gaseous mercury helide HeHg in 1925 HgHe₁₀; publishing the results in Nature, but then had trouble finding a stable composition, and eventually gave up.

Between 1925 and 1940 in Buenos Aires, Horacio Damianovich studied various metal-helium combinations including beryllium (BeHe), iron (FeHe), palladium (PdHe), platinum (Pt₃He, bismuth, and uranium. To make these substances, electrical discharges impacted helium into the surface of the metal. Later these were demoted from the status of compounds, to that of alloys. Platinum helide Pt₃He was discredited by J. G. Waller in 1960. Palladium helide PdHe is formed from tritium decay in palladium tritide, the helium (³He) is retained in the solid as a solution. Boomer claimed the discovery of Tungsten helide WHe₂ as a black solid. It is formed by way of an electric discharge in helium with a heated tungsten filament. When dissolved in nitric acid or potassium hdroxide, tungstic acid forms and helium escapes in bubbles. The electric discharge had a current of 5 ma and 1000 V at a pressure between 0.05 and 0.5 mm Hg for the helium. Functional electrolysis currents are from 2-20 ma and 5-10 ma works best. The process works slowly at 200 V. and 0.02 mm Hg of Hg vapour accelerates W evaporation by five times. The search for this was suggested by Ernest Rutherford. It was discredited by J. G. Waller in 1960. Boomer also studied Hg, I, S and P combinations with helium. Mercury and iodine helium combinations decomposed around -70 °C Sulfur and phosphorus helium combinations decomposed around -120 °C

• BiHe2

H. Krefft and R. Rompe claimed reactions between helium and sodium. potassium, zinc, rubidium, indium, and thallium.