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.^[1][2] 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.^[1][2]

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:

• 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.^[3]

• 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)

• 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)

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

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

• carbonium ions, protonated hydrocarbons, having a pentavalent 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 [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, C_nH+2n+1 (n >= 2) (protonated alkenes)
    • ethenium, C₂H+5 (protonated ethene)
  • alkynium cations, C_nH+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, Si_nH+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)

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

• 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)

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

• 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.)

• 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⁺

• 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)

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)

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

• Overview at chem.qmul.ac.uk
• Onium compounds at the US National Library of Medicine Medical Subject Headings (MeSH)

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