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 is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not join with anything to make covalent compounds. 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.

Known high pressure phases

• HeNa₂ known^[1]
• La_2/3-xLi_3xTiO₃He (like a clathrate)^[2]
• crystobalite He II (SiO₂He) between 1.7 and 6.4 Gpa rhombohedral space group R-3c a=9.080 α=31.809° V=184.77 Å₃ at 4GPa ^[3]
• crystobalite He I (SiO₂He) over 6.4 Gpa monoclinic space group P2₁/C with a=8.062 b=4.797 c=9.491 Å β=120.43° V=316.47 Å₃ at 10 Gpa^[4]
• silica glass helium: Helium penetrates into silica glass and reduces its compressibility.^[5]
• 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.^[6]
• 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, ^[7]stable to over 90 Gpa.^[8]
• dihelium arsenolite As₄O₆•2He over 5 GPa and up to at least 30 GPa. Arsenolite is one of the softest and most compressible minerals.^[9]

Known van der Waals molecules

• LiHe^[10]
• dihelium
• trihelium
• Ag³He^[11]
• HeCO is weakly bound by van der Waals forces. It is potentially important in cold interstellar media as both CO and He are common.^[12]

Known ions

• HeH⁺
• PtHe²⁺;^[13][14] formed by high electric field off platinum surface in helium.^[15]
• VHe^{2+ [15]}
• XeHe^{2+ [15]}
• KrHe^{2+ [15]}
• ArHe^{2+ [15]}
• HeRh²⁺ exists, decomposed in high strength electric field ^[16]
• RhHe^{2+ [17]}
• He₃^+[18] is in equilibrium with He₂⁺ between 135 and 200K^[19]
• HeN₂⁺ can form around 4K from an ion beam of N₂⁺ into cold helium gas.^[20]
• C₆₀He⁺ Formed by irradiating C₆₀ mwith 50eV electrons and then steering ions into cold helium gas.^[21]
• C₆₀He₂^+[21]

Predicted solids

• He(H₂O)₂ orthorhomic structure Ibam ^[22]

Predicted van der Waals molecules

• HeBeO OBeHe^[23]
• HeBe₂O₂ HeBe₂O₂He^[23]
• RNBeHe CH₃NBeHe^[23]
• HeNe
• HeCuF^[24] 10.1016/j.cplett.2009.10.010
• HeAgF unstable^[24]
• HeAuF predicted^[24]
• HHeF lifetime 157 femto seconds 05 kcal/mol barrier.^[24]
• HeNaO predicted
• C₈He predicted with He inside cube ^[2]
• Ag³He binding energy 1.4 cm^−1[25]
• Ag⁴He binding energy 1.85 cm⁻¹
• Au³He binding energy 4.91 cm^−1[25]
• Au⁴He binding energy 5.87 cm^−1[25]
• Li⁴He binding energy 0.008 cm⁻¹, the ³He is not stable.^[25]
• Na⁴He binding energy 0.03 cm⁻¹, the ³He is not stable.^[25]
• Cu³He binding energy 0.90 cm^−1[25]
• O⁴He binding energy 5.83 cm^−1[25]
• S⁴He binding energy 6.34 cm^−1[25]
• Se⁴He binding energy 6.50 cm^−1[25]
• F⁴He binding energy 3.85 cm^−1[25]
• Cl⁴He binding energy 7.48 cm^−1[25]
• Br⁴He binding energy 7.75 cm^−1[25]
• I⁴He binding energy 8.40 cm^−1[25]
• N⁴He binding energy 2.85 cm^−1[25]
• P⁴He binding energy 3.42 cm^−1[25]
• As⁴He binding energy 3.49 cm^−1[25]
• Bi⁴He binding energy 33.26 cm^−1[25]
• Si⁴He binding energy 1.95 cm^−1[25]
• Ge⁴He binding energy 2.08 cm^−1[25]
• CaH⁴He binding energy 0.96 cm^−1[25]
• NH⁴He binding energy 4.42 cm^−1[25]
• MnH⁴He binding energy 1.01 cm^−1[25]
• YbF⁴He binding energy 5.57 cm^−1[25]
• I₂⁴He or I₂³He^[26]

Predicted ions

Fluoroheliate ion

• Positronium Helide ion PsHe^{+ [27]}
• Fluoroheliate FHeO^- but salts like LiFHeO are not stable.^[28][18]
• HHeCO⁺ theoretical^[29]
• FHeS- predicted stable see doi=^[30]
• FHeBN⁻
• HRgN²⁺ unlikely ^[31]
• (HNg⁺)(OH2)(HNg⁺)(OH2) probably unstable ^[32]
• HLiHe⁺ lithium hydrohelide cation, linear in theory; this molecular ion could exist with big bang nucleosynthsis elements ^[33]
• HNaHe⁺ sodium hydrohelide cation ^[33]
• HKHe⁺ potassium hydrohelide cation ^[33]
• HBeHe²⁺ berylium hydrohelide cation ^[33]
• HMgHe²⁺ magnesium hydrohelide cation ^[33]
• HCaHe²⁺ calcium hydrohelide cation ^[33]
• HeY³⁺ predicted to be the lightest stable diatomic triply charged ion.^[34]
• HCHe^+[18]
• HCHeHe^+[18]
• HeF^-[18]
• HeO^-[18]
• HeS^-[18]
• FHeS^-[18]
• FHeSe^-[18]
• C₇H₆He^2+[18]
• C₇H₆HeHe^2+[18]
• FHeCC^−[18]
• HHeOH₂^+[18]
• HHeBF^+[18]
• HeNC^+[18]
• HeNN^+[18]
• HHeNN⁺ H-He 0.765 Å He-N bond length 2.077 Å. Decomposition barrier of 2.3 kJ/mol.^[18]
• 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^[18]
• He₂H^+[35]

Discredited or unlikely observations

• mercury helide HeHg^[36][37][38] HgHe₁₀;^[39][40]
• platinum helide Pt₃He was discredited by J. G. Waller in 1960.^[41]
• palladium helide PdHe is formed from palladium tritide radioactive decay, the helium is retained in the solid.
• tungsten helide WHe₂ is a black solid.^[42] 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. Electric discharge at 5 ma and 1000 V at between 0.05 and 0.5 mm Hg of pressure 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 5×. The search for this was suggested by Ernest Rutherford. It was discredited by J. G. Waller in 1960.^[41]
• FeHe iron helide is likely to be an interstitial compound.^[43] It perhaps can exist in dense planetary cores.^[44]
• BiHe2 ^[45][46][47]
• Mercury and iodine helium combinations decompose around -70 °C^[48]
• Sulfur and phosphorus helium combinations decompose around -120 °C^[48]

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