Onium compounds

Onium compounds are cations derived by the protonation of mononuclear parent hydrides of elements of the nitrogen group (Group 15), chalcogens (Group 16), or halogens (Group 17), and similar cations derived by the substitution of hydrogen atoms in the former by other groups, such as organic radicals, or halogens, for example tetramethylammonium, and further derivatives having polyvalent additions, such as iminium and nitrilium. (IUPAC definition [1]; other definitions vary.)

They are also known as onium ions. Compounds of these ions are known as onium salts. Onium ions have a charge of +1; double onium ions have a charge of +2, and so on.

Simple onium cations

• Group 15 (nitrogen group) onium cations:
  • ammonium, a.k.a. azanium, NH+
    4 (protonated ammonia, a.k.a. azane)
  • phosphonium, PH+
    4 (protonated phosphine)
  • arsonium, AsH+
    4 (protonated arsine)
  • stibonium, SbH+
    4 (protonated stibine)
  • bismuthonium, BiH+
    4 (protonated bismuthine)

• Group 16 (chalcogen) onium cations:
  • oxonium, better known as hydronium, H₃O⁺ (protonated water) (Hydronium implies a solvated or hydrated proton.) (May be called hydroxonium for disambiguation.)
  • sulfonium, H₃S⁺ (protonated hydrogen sulfide)
  • selenonium, H₃Se⁺ (protonated hydrogen selenide)
  • telluronium, H₃Te⁺ (protonated hydrogen telluride)

• 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)

• Group 14 (carbon group) onium cations (these ions use similar naming schemes. they do not strictly meet the IUPAC definition above):
  • methanium, CH+
    5 (protonated methane) (Sometimes methonium, but it has multiple definitions.) (Should not be called carbonium[2].) (Abundant in outer space.)
  • further alkanium cations, C_nH_(2n+3)⁺
    • ethanium, C₂H+
      7 (protonated ethane)
    • propanium, C₃H+
      9 (propane protonated on an unspecified carbon)
      • propylium, a.k.a. propan-1-ylium (propane protonated on an end carbon)
      • propan-2-ylium (propane protonated on the middle carbon)
  • silanium (sometimes silonium), SiH+
    5 (protonated silane) (Should not be called siliconium.[3])
  • germonium, GeH+
    5 (protonated germane)
  • stannonium, SnH+
    3 (protonated SnH+
    2; not protonated stannane SnH₄)

• Group 13 (boron group) onium cations (these ions use a similar naming scheme. they do not strictly meet the IUPAC definition above):
  • boronium cations, BH+
    3, BH+
    4, BH+
    5, BH+
    6 (protonated boranes)

• Hydrogen onium cation (this ion uses a similar naming scheme. it does not strictly meet the IUPAC definition above):
  • hydrogenonium, better known as trihydrogen cation, H+
    3 (protonated [molecular] or [diatomic] hydrogen). (Abundant in outer space.)

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, a.k.a. 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

• quaternary phosphonium cations, R₄P⁺ or NP+
  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⁺

Onium cations with polyvalent substitutions

• secondary ammonium cations having one double-bonded substitution, R=NH₂⁺
  • diazenium, HN=NH₂⁺ (protonated diazene)

• 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)

• 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)

• ternary oxonium cations having one triple-bonded substitution, R≡O⁺
  • nitrosonium, N≡O⁺

• tertiary sulfonium cations having one triple-bonded substitution, R≡S⁺
  • thionitrosyl, N≡S⁺

Double-onium dications

• hydrazinium dication, ⁺H₃NNH₃⁺ (doubly protonated hydrazine)
• diazenium dication, ⁺H₂N=NH₂⁺ (doubly protonated diazene)
• diazonium dication, ⁺HN≡NH⁺ (doubly protonated nitrogen)

External links

• Onium compounds at IUPAC
• Onium compounds at IUPAC
• Overview at chem.qmul.ac.uk
• MeSH Onium+compounds