Onium ion

Not to be confused with onium states such as positronium.

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 groups, 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 anion are known as onium compounds or onium salts.

Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:

• Lewis bases form onium ions when the central atom gains one more bond and becomes a positive cation.
• Lewis acids form -ate ions when the central atom gains one more bond and becomes a negative anion.

Simple onium cations (hydrides with no substitutions)

Group 13 (boron group) onium cations

• boronium cation, BH+4 (protonated borane)
  • further boronium cations, B
    _xH⁺
    _y (protonated boranes)

Group 14 (carbon group) onium cations

• carbonium ions (protonated hydrocarbons) have a pentacoordinated carbon atom with a +1 charge.
  • alkanium cations, C
    _nH⁺
    _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. 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)
• silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium
  • disilanium, Si₂H+7 (protonated disilane)
  • further silanium cations, Si
    _nH⁺
    _2n+3 (protonated silanes)
• germonium, GeH+5 (protonated germane)
• stannonium, SnH+3 (protonated SnH₂) (not protonated stannane SnH₄)
• plumbonium, PbH+3 (protonated PbH₂) (not protonated plumbane PbH₄)
• flerovonium, FlH+3 (protonated FlH₂) (not protonated flerovane FlH₄)

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)
• moscovonium, McH+4 (protonated moscovine)

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)
• polononium, H₃Po⁺ (protonated hydrogen polonide)
• livermoronium, H₃Lv⁺ (protonated hydrogen livermoride)

Hydrogen onium cation

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

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)
• astatonium, H₂At⁺ (protonated hydrogen astatide)
• tennessonium, H₂Ts⁺ (protonated hydrogen tennesside)

Pseudohalogen onium cations

• aminodiazonium, [H₂N=N=N]⁺ ⇌ [H₂N−N≡N]⁺ (protonated hydrogen azide)
• methylidyneammonium and hydrocyanonium, H₂CN⁺, isomers HC≡NH⁺ ⇌ N≡CH+2 (protonated hydrogen cyanide)

Group 18 (noble gas) onium cations

• hydrohelium or helonium, better known as helium hydride ion, HeH⁺ (protonated helium)
• neonium, NeH⁺ (protonated neon)
• argonium, ArH⁺ (protonated argon)
• kryptonium, KrH⁺ (protonated krypton)
• xenonium, XeH⁺ (protonated xenon)
• radonium, RnH⁺ (protonated radon)
• oganessonium OgH⁺ (protonated oganesson)

Onium cations with monovalent substitutions

• primary ammonium cations, RH₃N⁺ or R−NH+3 (protonated primary amines)
  • hydroxylammonium, H₃N⁺−OH (protonated hydroxylamine)
  • methylammonium, CH₃NH+3 (protonated methylamine)
  • ethylammonium, CH₃CH₂NH+3 (protonated ethylamine)
  • hydrazinium, or diazanium, H₂N−NH+3 (protonated hydrazine, a.k.a. diazane)
  • anilinium (a.k.a. phenylammonium), C₆H₅−NH+3 (protonated aniline, a.k.a. phenylamine, aminobenzene)
• secondary ammonium cations, R₂NH+2 (protonated secondary amines)
  • dimethylammonium (sometimes dimethylaminium), (CH₃)₂NH+2 (protonated dimethylamine)
  • diethylammonium (sometimes diethylaminium), (CH₃CH₂)₂NH+2 (protonated diethylamine)
  • ethylmethylammonium, (CH₃CH₂)(CH₃)NH+2 (protonated ethylmethylamine)
  • diethanolammonium (sometimes diethanolaminium), (OHCH₂CH₂)₂NH+2 (protonated diethanolamine)
• tertiary ammonium cations, R₃NH⁺ (protonated tertiary amines)
  • trimethylammonium (CH₃)₃NH⁺ (protonated trimethylamine)
  • triethylammonium (CH₃CH₂)₃NH⁺ (protonated triethylamine)
• quaternary ammonium cations, R₄N⁺ or NR+4
  • tetrafluoroammonium, NF+4
  • tetramethylammonium, (CH₃)₄N⁺
  • tetraethylammonium, (CH₃CH₂)₄N⁺
  • tetrapropylammonium, (CH₃(CH₂)₂)₄N⁺
  • tetrabutylammonium, (CH₃(CH₂)₃)₄N⁺ or abbreviated Bu₄N⁺
  • trimethyl ammonium compounds, (CH₃)₃RN⁺
  • didecyldimethylammonium, (CH₃(CH₂)₉)₂(CH₃)₂N⁺
  • pentamethylhydrazinium, (CH₃)₂N−N(CH₃)+3
• quaternary phosphonium cations, R₄P⁺ or PR+4
  • tetraphenylphosphonium, (C₆H₅)₄P⁺
• quaternary arsonium cations, R₄As⁺ or AsR+4
  • tetraphenylarsonium, (C₆H₅)₄As⁺
• quaternary stibonium cations, R₄Sb⁺ or SbR+4
  • tetraphenylstibonium, (C₆H₅)₄Sb⁺
• primary oxonium cations, ROH+2 (protonated alcohols R−O−H)
  • alkyloxonium cations ROH+2 (protonated alcohols)
    • methyloxonium, CH₃OH+2 (protonated methanol)
    • ethyloxonium, CH₃CH₂OH+2 (protonated ethanol)
  • dioxidanonium (hydroxylhydronium), HO−OH+2 (protonated hydrogen peroxide)
• secondary oxonium cations, R₂OH⁺ (protonated ethers R−O−R)
  • dialkyloxonium cations (protonated ethers)
    • dimethyloxonium, (CH₃)₂OH⁺ (protonated dimethyl ether)
• tertiary oxonium cations, R₃O⁺
  • trifluorooxonium, OF+3 (hypothetical)
  • trimethyloxonium, (CH₃)₃O⁺
  • triethyloxonium, (CH₃CH₂)₃O⁺
  • oxatriquinacene, C₉H₉O⁺ (cyclic oxonium ion)
  • oxatriquinane, C₉H₁₅O⁺ (cyclic oxonium ion)
• primary sulfonium cations, RSH+2 (protonated thiols R−S−H)
• secondary sulfonium cations, R₂SH⁺ (protonated thioethers R−S−R)
  • dimethylsulfonium, (CH₃)₂SH⁺ (protonated dimethyl sulfide)
• tertiary sulfonium cations, R₃S⁺
  • trimethylsulfonium, (CH₃)₃S⁺
• tertiary selenonium cations, R₃Se⁺
  • triphenylselenonium, (C₆H₅)₃Se⁺
• tertiary telluronium cations, R₃Te⁺
  • triphenyltelluronium, (C₆H₅)₃Te⁺
• primary fluoronium cations, RFH⁺ (protonated fluorides RF)
• secondary fluoronium cations, R₂F⁺
  • dichlorofluoronium, Cl₂F⁺
• secondary iodonium cations, R₂I⁺
  • diphenyliodonium, (C₆H₅)₂I⁺

Onium cations with polyvalent substitutions

• secondary ammonium cations having one double-bonded substitution, R=NH+2
  • diazenium, HN=NH+2 (protonated diazene)
  • guanidinium, C(NH₂)+3 (protonated guanidine) (has a resonance structure and a planar molecular geometry)
• tertiary ammonium cations having one triple-bonded substitution, R≡NH+
  • nitrilium, R−C≡NH⁺ (protonated nitrile)
  • diazonium or diazynium, N≡NH⁺ (protonated nitrogen, in other words, protonated diazyne)
• cyclic tertiary ammonium cations where nitrogen is a member of a ring, RNH⁺R (the ring may be aromatic)
  • pyridinium, C₅H₅NH⁺ (protonated pyridine)
• quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR+2
  • iminium, R₂C=NR+2 (substituted protonated imine)
  • diazenium, RN=NR+2 (substituted protonated diazene)
  • thiazolium, [C₃NSR₄]⁺(substituted protonated thiazole)
• quaternary ammonium cations having two double-bonded substitutions, R=N⁺=R
  • nitronium, [NO₂]⁺
  • bis(triphenylphosphine)iminium, ((C₆H₅)₃P=)₂N⁺
• quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR⁺
  • diazonium, N≡NR⁺ (substituted protonated nitrogen, in other words, substituted protonated diazyne)
  • nitrilium, RC≡NR⁺ (substituted protonated nitrile)
• tertiary oxonium cations having one triple-bonded substitution, R≡O⁺
  • acylium ions, R−C≡O⁺ ↔ R−C⁺=O
  • nitrosonium, N≡O⁺
• cyclic tertiary oxonium cations where oxygen is a member of a ring, RO⁺R (the ring may be aromatic)
  • pyrylium, C₅H₅O⁺
• tertiary sulfonium cations having one triple-bonded substitution, R≡S⁺
  • thionitrosyl, N≡S⁺
• dihydroxyoxoammonium, [H₂NO₃]⁺ (protonated nitric acid)
• trihydroxyoxosulfonium, [H₃SO₄]⁺ (protonated sulfuric acid)

Double onium dications

• hydrazinediium or hydrazinium(2+) dication, H₃N⁺−⁺NH₃ (doubly protonated hydrazine, in other words, doubly protonated diazane)
• diazenediium cation, H₂N⁺=⁺NH₂ (doubly protonated diazene)
• diazynediium cation, HN⁺≡⁺NH (doubly protonated dinitrogen, in other words, doubly protonated diazyne)

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 tricoordinated carbon atom with a +1 charge.
  • alkenium cations, C
    _nH⁺
    _2n+1 (n ≥ 2) (protonated alkenes)
    • methenium cation, H₃C⁺ (protonated methylene)
    • ethenium, C₂H+5 (protonated ethene)
  • benzenium, C₆H+7 (protonated benzene)
  • tropylium, C₇H+7 (protonated tropylidene)
• 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)

Substituted eniums

• diphenylcarbenium, (C₆H₅)₂CH⁺ (di-substituted methenium)
• triphenylcarbenium, (C₆H₅)₃C⁺ (tri-substituted methenium)

Ynium cations

• carbynium ions (protonated carbynes) have a carbon atom with a +1 charge.
  • alkynium cations, C
    _nH⁺
    _2n-1 (n ≥ 2) (protonated alkynes)
    • methynium cation, H₂C⁺ (protonated methylidyne radical)
    • ethynium, C₂H+3 (protonated ethyne)

See also

• Carbonium ion
• Lyonium ion, a protonated solvent molecule
• Lyate ion, a deprotonated solvent molecule

External links

• Ions and Radicals, Queen Mary University of London
• Onium compounds at the U.S. National Library of Medicine Medical Subject Headings (MeSH)