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HandWiki. Onium Compound. Encyclopedia. Available online: (accessed on 20 June 2024).
HandWiki. Onium Compound. Encyclopedia. Available at: Accessed June 20, 2024.
HandWiki. "Onium Compound" Encyclopedia, (accessed June 20, 2024).
HandWiki. (2022, November 16). Onium Compound. In Encyclopedia.
HandWiki. "Onium Compound." Encyclopedia. Web. 16 November, 2022.
Onium Compound

In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, NH+4, the protonated derivative of ammonia, NH3. The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic radicals, or halogens; such as tetraphenylphosphonium, (C6H5)4P+. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium. A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on. Compounds of an onium cation and some other negative ion are known as onium compounds or onium salts. Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:

tetraphenylphosphonium nitrilium chalcogen

1. Simple Onium Cations (Hydrides with No Substitutions)

1.1. Group 15 (Pnictogen) Onium Cations

  • ammonium (IUPAC name azanium), NH+4 (protonated ammonia (IUPAC name azane))
  • phosphonium, PH+4 (protonated phosphine)
  • arsonium, AsH+4 (protonated arsine)
  • stibonium, SbH+4 (protonated stibine)
  • bismuthonium, BiH+4 (protonated bismuthine)

1.2. Group 16 (Chalcogen) Onium Cations

  • oxonium, H3O+ (protonated water (IUPAC name oxidane). Oxonium is better known as hydronium, though hydronium implies a solvated or hydrated proton. It may also be called hydroxonium.)
  • sulfonium, H3S+ (protonated hydrogen sulfide)
  • selenonium, H3Se+ (protonated hydrogen selenide)
  • telluronium, H3Te+ (protonated hydrogen telluride)

1.3. Group 17 (Halogen) Onium Cations, Halonium Ions, H2X+ (Protonated Hydrogen Halides)

  • fluoronium, H2F+ (protonated hydrogen fluoride)
  • chloronium, H2Cl+ (protonated hydrogen chloride)
  • bromonium, H2Br+ (protonated hydrogen bromide)
  • iodonium, H2I+ (protonated hydrogen iodide)

Pseudohalogen onium cations

  • aminodiazonium, H2N+3 (protonated hydrogen azide)
  • hydrocyanonium, H2CN+ (protonated hydrogen cyanide)

1.4. Group 14 (Carbon Group) Onium Cations

  • carbonium ions, protonated hydrocarbons, having a pentavalent carbon atom with a +1 charge
    • alkanium cations, CnH+2n+3 (protonated alkanes)
      • methanium, CH+5 (protonated methane) (Sometimes called carbonium, because it is the simplest member of that class, but that use is deprecated because of multiple definitions [1]. Sometimes called methonium, but methonium also has multiple definitions. Abundant in outer space.)
      • ethanium, C2H+7 (protonated ethane)
      • propanium, C3H+9 (propane protonated on an unspecified carbon)
        • propylium, or propan-1-ylium (propane protonated on an end carbon)
        • propan-2-ylium (propane protonated on the middle carbon)
      • butanium, C4H+11 (butane protonated on an unspecified carbon)
        • n-butanium (n-butane protonated on an unspecified carbon)
          • n-butylium, or n-butan-1-ylium (n-butane protonated on an end carbon)
          • n-butan-2-ylium (n-butane protonated on a middle carbon)
        • isobutanium (isobutane protonated on an unspecified carbon)
          • isobutylium, or isobutan-1-ylium (isobutane protonated on an end carbon)
          • isobutan-2-ylium (isobutane protonated on the middle carbon)
      • octonium or octanium, C8H+19 (protonated octane)
    • alkenium cations, CnH+2n+1 (n >= 2) (protonated alkenes)
      • ethenium, C2H+5 (protonated ethene)
    • alkynium cations, CnH+2n-1 (n >= 2) (protonated alkynes)
      • ethynium, C2H+3 (protonated ethyne)
  • silanium (sometimes silonium), SiH+5 (protonated silane. Should not be called siliconium.[2])
  • further silanium cations, SinH+2n+3 (protonated silanes)
    • disilanium, Si2H+7 (protonated disilane)
  • germonium, GeH+5 (protonated germane)
  • stannonium, SnH+3 (protonated SnH2; not protonated stannane SnH4)
  • plumbonium, PbH+3 (protonated PbH2)

1.5. Group 13 (Boron Group) Onium Cations

  • boronium cation, BH+4 (protonated borane)
  • further boronium cations, BxH+y (protonated boranes)

1.6. Group 18 (Noble Gas) Onium Cations

  • hydrohelium or helonium, better known as helium hydride ion, HeH+ (protonated helium), expected in outer space, but not yet detected
  • neonium, NeH+ (protonated neon)
  • argonium, ArH+ (protonated argon)
  • kryptonium, KrH+ (protonated krypton)
  • xenonium, XeH+ (protonated xenon)

1.7. Hydrogen Onium Cation

  • hydrogenonium, better known as trihydrogen cation, H+3 (protonated [molecular] or [diatomic] hydrogen), found in ionized hydrogen and interstellar space

2. Onium Cations with Monovalent Substitutions

  • primary ammonium cations, RH3N+ or RNH+3 (protonated primary amines)
    • hydroxylammonium, NH3OH+ (protonated hydroxylamine)
    • methylammonium, CH3NH+3 (protonated methylamine)
    • ethylammonium, C2H5NH+3 (protonated ethylamine)
    • hydrazinium, or diazanium, NH2NH+3 (protonated hydrazine, a.k.a. diazane)
  • secondary ammonium cations, R2NH+2 (protonated secondary amines)
    • dimethylammonium (sometimes dimethylaminium), (CH3)2NH+2 (protonated dimethylamine)
    • diethylammonium (sometimes diethylaminium), (C2H5)2NH+2 (protonated diethylamine)
    • ethylmethylammonium, C2H5CH3NH+2 (protonated ethylmethylamine)
    • diethanolammonium (sometimes diethanolaminium), (C2H4OH)2NH+2 (protonated diethanolamine)
  • tertiary ammonium cations, R3NH+ (protonated tertiary amines)
    • trimethylammonium (CH3)3NH+ (protonated trimethylamine)
    • triethylammonium (C2H5)3NH+ (protonated triethylamine)
  • quaternary ammonium cations, R4N+ or NR+4
    • tetramethylammonium, (CH3)4N+
    • tetraethylammonium, (C2H5)4N+
    • tetrapropylammonium, (C3H7)4N+
    • tetrabutylammonium, (C4H9)4N+ or abbreviated Bu4N+
    • trimethyl ammonium compounds, (CH3)3RN+
    • didecyldimethylammonium, (C10H21)2(CH3)2N+
    • pentamethylhydrazinium, N(CH3)2N(CH3)+3
    • tetrafluoroammonium, NF+4
  • quaternary phosphonium cations, R4P+ or PR+4
    • tetraphenylphosphonium, (C6H5)4P+
  • secondary sulfonium cations, R2SH+ (protonated sulfides)
    • dimethylsulfonium, (CH3)2SH+ (protonated dimethyl sulfide)
  • tertiary sulfonium cations, R3S+
    • trimethylsulfonium, (CH3)3S+
  • secondary fluoronium cations, R2F+
    • dichlorofluoronium, Cl2F+
  • trifluoroxenonium, XeF+3 (XeF2 is neutral.)

3. Onium Cations with Polyvalent Substitutions

  • secondary ammonium cations having one double-bonded substitution, R=NH2+
    • diazenium, HN=NH2+ (protonated diazene)
    • guanidinium, (H2N)2C=NH2+ (protonated guanidine) (has a resonance structure)
  • quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR2+
    • iminium, R2C=NR2+ (substituted protonated imine)
    • diazenium, RN=NR2+ (substituted protonated diazene)
  • quaternary ammonium cations having two double-bonded substitutions, R=N=R+
    • nitronium, NO+2
    • bis(triphenylphosphine)iminium, ((C6H5)3P)2N+
  • tertiary ammonium cations having one triple-bonded substitution, R≡NH+
    • nitrilium, RC≡NH+ (protonated nitrile)
    • diazonium, N≡NH+ (protonated nitrogen)
  • tertiary ammonium cations having two partially double-bonded substitutions, RNH+R
    • pyridinium, C5H5NH+ (protonated pyridine)
  • quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR+
    • diazonium, N≡NR+ (substituted protonated nitrogen)
    • nitrilium, RC≡NR+ (substituted protonated nitrile)
  • tertiary oxonium cations having one triple-bonded substitution, R≡O+
    • nitrosonium, N≡O+
  • tertiary oxonium cations having two partially double-bonded substitutions, RO+R
    • pyrylium, C5H5O+
  • tertiary sulfonium cations having one triple-bonded substitution, R≡S+
    • thionitrosyl, N≡S+

4. Double Onium Dications

  • hydrazinediium or hydrazinium(2+) dication, +H3NNH+3 (doubly protonated hydrazine)
  • diazenium dication, +H2N=NH+2 (doubly protonated diazene)
  • diazonium dication, +HN≡NH+ (doubly protonated [di]nitrogen)

5. Enium Cations

The extra bond is added to a less-common parent hydride, a carbene analog, typically named -ene or -ylene, which is neutral with 2 fewer bonds than the more-common hydride, typically named -ane or -ine.

  • borenium cations, R2B+ (protonated borylenes a.k.a. boranylidenes)
  • carbenium cations, R3C+ (protonated carbenes, have a trivalent carbon atom with a +1 charge)
    • methenium cation, H3C+ (protonated methylene)
  • silylium cations, R3Si+ (protonated silylenes)
  • nitrenium cations, R2N+ (protonated nitrenes)
  • phosphinidenium cations, R2P+ (protonated phosphinidene)
  • mercurinium cations, R3Hg+ (protonated organomercury compounds; formed as intermediates in oxymercuration reactions)

6. Ynium Cations

  • carbynium ions, protonated carbynes, have a divalent carbon atom with a +1 charge
    • methynium cation, H2C+ (protonated methylidyne radical)

7. External Links

  • Overview at
  • Onium compounds at the US National Library of Medicine Medical Subject Headings (MeSH)
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