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, NH
3.^[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, (C
6H
5)
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]

Simple onium cations (hydrides with no substitutions)

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)

Group 16 (chalcogen) onium cations

• oxonium, H
  3O+
  (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, H
  3S+
  (protonated hydrogen sulfide)
• selenonium, H
  3Se+
  (protonated hydrogen selenide)
• telluronium, H
  3Te+
  (protonated hydrogen telluride)

Group 17 (halogen) onium cations, halonium ions, H₂X⁺ (protonated hydrogen halides)

• fluoronium, H
  2F+
  (protonated hydrogen fluoride)
• chloronium, H
  2Cl+
  (protonated hydrogen chloride)
• bromonium, H
  2Br+
  (protonated hydrogen bromide)
• iodonium, H
  2I+
  (protonated hydrogen iodide)

Pseudohalogen onium cations

• aminodiazonium, H
  2N+
  3 (protonated hydrogen azide)
• hydrocyanonium, H
  2CN+
  (protonated hydrogen cyanide)

Group 14 (carbon group) onium cations

All are carbonium ions.

• 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, C
    2H+
    7 (protonated ethane)
  • propanium, C
    3H+
    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, C
    4H+
    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, C
    8H+
    19 (protonated octane)
• alkenium cations, C
  nH+
  2n+1 (n >= 2) (protonated alkenes)
  • ethenium, C
    2H+
    5 (protonated ethene)
• alkynium cations, C
  nH+
  2n-1 (n >= 2) (protonated alkynes)
  • ethynium, C
    2H+
    3 (protonated ethyne)
• silanium (sometimes silonium), SiH+
  5 (protonated silane. Should not be called siliconium.)
• further silanium cations, Si
  nH+
  2n+3 (protonated silanes)
  • disilanium, Si
    2H+
    7 (protonated disilane)
• germonium, GeH+
  5 (protonated germane)
• stannonium, SnH+
  3 (protonated SnH
  2; not protonated stannane SnH
  4)
• plumbonium, PbH+
  3 (protonated PbH
  2)

Group 13 (boron group) onium cations

• boronium cation, BH+
  3 (protonated borane)
• further boronium cations, B
  xH+
  y (protonated boranes)

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)

Hydrogen onium cation

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

Onium cations with monovalent substitutions

• primary ammonium cations, RH
  3N+
  or RNH+
  3 (protonated primary amines)
  • hydroxylammonium, NH
    3OH+
    (protonated hydroxylamine)
  • methylammonium, CH
    3NH+
    3 (protonated methylamine)
  • ethylammonium, C
    2H
    5NH+
    3 (protonated ethylamine)
  • hydrazinium, or diazanium, NH
    2NH+
    3 (protonated hydrazine, a.k.a. diazane)
• secondary ammonium cations, R
  2NH+
  2 (protonated secondary amines)
  • dimethylammonium (sometimes dimethylaminium), (CH
    3)
    2NH+
    2 (protonated dimethylamine)
  • diethylammonium (sometimes diethylaminium), (C
    2H
    5)
    2NH+
    2 (protonated diethylamine)
  • ethylmethylammonium, C
    2H
    5CH
    3NH+
    2 (protonated ethylmethylamine)
  • diethanolammonium (sometimes diethanolaminium), (C
    2H
    4OH)
    2NH+
    2 (protonated diethanolamine)
• tertiary ammonium cations, R
  3NH+
  (protonated tertiary amines)
  • trimethylammonium (CH
    3)
    3NH+
    (protonated trimethylamine)
  • triethylammonium (C
    2H
    5)
    3NH+
    (protonated triethylamine)
• quaternary ammonium cations, R
  4N+
  or NR+
  4
  • tetramethylammonium, (CH
    3)
    4N+
    
  • tetraethylammonium, (C
    2H
    5)
    4N+
    
  • tetrapropylammonium, (C
    3H
    7)
    4N+
    
  • tetrabutylammonium, (C
    4H
    9)
    4N+
    or abbreviated Bu
    4N+
    
  • trimethyl ammonium compounds, (CH
    3)
    3RN+
    
  • didecyldimethylammonium, (C
    10H
    21)
    2(CH
    3)
    2N+
    
  • pentamethylhydrazinium, N(CH
    3)
    2N(CH
    3)+
    3
  • tetrafluoroammonium, NF+
    4
• quaternary phosphonium cations, R
  4P+
  or PR+
  4
  • tetraphenylphosphonium, (C
    6H
    5)
    4P+
    
• secondary sulfonium cations, R
  2SH+
  (protonated sulfides)
  • dimethylsulfonium, (CH
    3)
    2SH+
    (protonated dimethyl sulfide)
• tertiary sulfonium cations, R
  3S+
  
  • trimethylsulfonium, (CH
    3)
    3S+
    
• secondary fluoronium cations, R
  2F+
  
  • dichlorofluoronium, Cl
    2F+
    
• trifluoroxenonium, XeF+
  3 (XeF
  2 is neutral.)

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, ((C
    6H
    5)
    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, C
    5H
    5NH+
    (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, C
    5H
    5O+
    
• 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 [di]nitrogen)

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, R
  2B+
  (protonated borylenes a.k.a. boranylidenes)
• carbenium cations, R
  3C+
  (protonated carbenes)
  • methenium cation, H
    3C+
    (protonated methylene)
• silylium cations, R
  3Si+
  (protonated silylenes)
• nitrenium cations, R
  2N+
  (protonated nitrenes)
• phosphinidenium cations, R
  2P+
  (protonated phosphinidene)
• mercurinium cations, R
  3Hg+
  (protonated organomercury compounds; formed as intermediates in oxymercuration reactions)

External links

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

References

1. IUPAC definition
2. George A. Olah (1998), Onium Ions. John Wiley & Sons, 509 pages. ISBN 9780471148777
3. Advanced Organic Chemistry: Reactions and mechanisms, Maya Shankar Singh, 2007, Dorling Kindersley, ISBN 978-81-317-1107-1