Onium ion

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 groups, 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 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.^[3]

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.^[4] 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^[5]
- 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)

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, 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)

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)

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)

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)

References

^ ^a ^b Onium compounds, IUPAC Gold Book
^ ^a ^b George A. Olah (1998). Onium Ions. John Wiley & Sons. p. 509. ISBN 9780471148777.
^ Advanced Organic Chemistry: Reactions and mechanisms, Maya Shankar Singh, 2007, Dorling Kindersley, ISBN 978-81-317-1107-1
^ Carbonium ion, IUPAC Gold Book
^ RC-82. Cations, Queen Mary University of London)

External links

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

[IUPAC-1] Onium compounds, IUPAC Gold Book

[OlahBk-2] George A. Olah (1998). Onium Ions. John Wiley & Sons. p. 509. ISBN 9780471148777.

[3] Advanced Organic Chemistry: Reactions and mechanisms, Maya Shankar Singh, 2007, Dorling Kindersley, ISBN 978-81-317-1107-1

[4] Carbonium ion, IUPAC Gold Book

[5] RC-82. Cations, Queen Mary University of London)

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