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Helium compounds

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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]
  • La2/3-xLi3xTiO3He (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]
  • VHe2+ [15]
  • XeHe2+ [15]
  • KrHe2+ [15]
  • ArHe2+ [15]
  • HeRh2+ exists, decomposed in high strength electric field [16]
  • RhHe2+ [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]
Wikimedia Commons has media related to Fluoroheliates.
  • 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]
  • C7H6He2+[18]
  • C7H6HeHe2+[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 C2v 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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