Onium ion

This set index article includes a list of related items that share the same name (or similar names).
If an internal link incorrectly led you here, you may wish to change the link to point directly to the intended article.

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.

Periodic table

Onium ions by group

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. The specific cation CH+5 is called methanium.. Typically named for the parent hydrocarbon, e.g. C₂H+7 is ethanium.
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) Unstable derivative known of R₃Ge⁺.
stannonium, SnH+3 (protonated stannylene, SnH₂) (not protonated stannane SnH₄) stable at cryogenic conditions.

Group 15 (pnictogen) onium cations

ammonium (IUPAC name azanium), NH+4 (protonated ammonia (IUPAC name azane)) exists in aqueous solution and as salts.
- primary, secondary, and tertiary organic derivatives [NH_nR_4−n]⁺, derived from protonation of amines; e.g. methylammonium halides, diethylammonium chloride, trimethylammonium
- quaternary ammonium cations NR+4; e.g. tetrabutylammonium
phosphonium, PH+4 (protonated phosphine)
- primary, secondary, and tertiary organic derivatives [PH_nR_4−n]⁺, derived from protonation of phosphines
- quaternary phosphonium cations PR+4; e.g. tetraphenylphosphonium
arsonium, AsH+4 (protonated arsine) known as derivatives or as an unstable salt at cryogenic temperatures.
stibonium, SbH+4 (protonated stibine) known as derivatives but no salts are known.
bismuthonium, BiH+4 (protonated bismuthine) only known as derivatives.

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.)
- Organic derivatives can be primary (ROH+2, protonated alcohols), secondary (R₂OH⁺, protonated ethers), or ternary (R₃O⁺, as trimethyloxonium).
peroxonium, H₃O+2 (protonated hydrogen peroxide)
sulfonium, H₃S⁺ (protonated hydrogen sulfide)
- Organic derivatives can be primary (H₂SR⁺, protonated thiols), secondary (HSR+2, protonated thioethers), or ternary (SR+3, e.g. trimethylsulfonium)
selenonium, H₃Se⁺ (protonated hydrogen selenide)
- Tertiary organic derivatives R₃Se⁺ are known, with trimethylselenonium iodide being the first.
telluronium, H₃Te⁺ (protonated hydrogen telluride)

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)

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) known in gas phase.
neonium, NeH⁺ (protonated neon) known in gas phase.
argonium, ArH⁺ (protonated argon) known in gas phase.
kryptonium, KrH⁺ (protonated krypton) known in gas phase.
xenonium, XeH⁺ (protonated xenon) known in gas phase.

Onium cations with monovalent substitutions

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 [di-]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 [di-]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⁺
tertiary sulfonium cations having one triple-bonded substitution, R≡S⁺
- thionitrosyl, N≡S⁺
dihydroxyoxoammonium, [H₂NO₃]⁺ (protonated nitric acid)
trihydroxyoxosulfonium, [H₃SO₄]⁺ (protonated sulfuric acid)
cyclic tertiary onium cations
- pyrylium, C₅H₅O⁺
- thiopyrylium, C₅H₅S⁺
- selenopyrylium, C₅H₅Se⁺
- telluropyrylium, C₅H₅Te⁺

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 [di-]nitrogen; 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

1 2 Onium compounds, IUPAC Gold Book
1 2 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
↑ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "carbonium ion". doi:10.1351/goldbook.C00839
↑ Yeh, L. I.; Price, J. M.; Lee, Yuan T. (July 1989). "Infrared spectroscopy of the pentacoordinated carbonium ion C2H7+". Journal of the American Chemical Society. 111 (15): 5597–5604. Bibcode:1989JAChS.111.5597Y. doi:10.1021/ja00197a015.
↑ RC-82. Cations, Queen Mary University of London)
↑ Weinert, Charles S. (March 2011). "Synthetic, Structural, and Physical Aspects of Organo-Oligogermanes". Comments on Inorganic Chemistry. 32 (2): 55–87. doi:10.1080/02603594.2011.618854.
↑ Sollott, Gilbert P.; Peterson, William R. (December 1967). "Germylation of ferrocene under Friedel-Crafts conditions. Question of the existence of germonium ions". Journal of the American Chemical Society. 89 (25): 6783–6784. Bibcode:1967JAChS..89.6783S. doi:10.1021/ja01001a082.
↑ Leighton, Kevin L.; Wasylishen, Roderick E. (1 July 1987). "Deuterium isotope effects on the ¹¹⁹Sn shielding constants and spin–spin coupling constants in stannane and the stannonium cation". Canadian Journal of Chemistry. 65 (7): 1469–1473. Bibcode:1987CaJCh..65.1469L. doi:10.1139/v87-250. ISSN 0008-4042.
1 2 Smith, J. D. (22 October 2013). The Chemistry of Arsenic, Antimony and Bismuth: Pergamon Texts in Inorganic Chemistry. Elsevier. p. 588. ISBN 978-1-4831-8754-9.
↑ Norman, N. C. (31 December 1997). Chemistry of Arsenic, Antimony and Bismuth. Springer Science & Business Media. pp. 310–323. ISBN 978-0-7514-0389-3.
↑ Leicester, Henry M.; Bergstrom, F. W. (1929). "Salts of Triphenylselenonium Hydroxide". Journal of the American Chemical Society. 51 (12): 3587–3591. Bibcode:1929JAChS..51.3587L. doi:10.1021/ja01387a011.
↑ Bainbridge, Kenneth T. (1 July 1933). "Comparison of the Masses of H₂ and Helium". Physical Review. 44 (1): 57. Bibcode:1933PhRv...44...57B. doi:10.1103/PhysRev.44.57.
↑ Kuntz, P. J.; Roach, A. C. (1972). "Ion-molecule reactions of the rare gases with hydrogen. Part 1.—Diatomics-in-molecules potential energy surface for ArH+2". J. Chem. Soc., Faraday Trans. 2. 68: 259–280. doi:10.1039/F29726800259.
↑ Neufeld, David A.; Wolfire, Mark G. (1 August 2016). "The Chemistry of Interstellar Argonium and Other Probes of the Molecular Fraction in Diffuse Clouds". The Astrophysical Journal. 826 (2): 183. arXiv:1607.00375. Bibcode:2016ApJ...826..183N. doi:10.3847/0004-637X/826/2/183.
↑ Linnartz, H.; Zink, L.R.; Evenson, K.M. (July 1997). "The Pure Rotational Spectra of ⁸⁴KrH⁺ and ⁸⁶KrH⁺". Journal of Molecular Spectroscopy. 184 (1): 56–59. Bibcode:1997JMoSp.184...56L. doi:10.1006/jmsp.1997.7297.
↑ Grandinetti, Felice (October 2011). "Gas-Phase Ion Chemistry of the Noble Gases: Recent Advances and Future Perspectives". European Journal of Mass Spectrometry. 17 (5): 423–463. doi:10.1255/ejms.1151.
↑ Mitcham, Renonia V.; Lee, Byungkook; Mertes, Kristin Bowman; Ziolo, Ronald F. (1 December 1979). "The nature of triphenylselenonium chloride. Crystal and molecular structure of the monohydrate: triphenylselenonium chloride hydrate". Inorganic Chemistry. 18 (12): 3498–3502. doi:10.1021/ic50202a041.
↑ Ziolo, R. F.; Titus, D. D. (1 December 1976). "Crystal data for triphenyl telluronium pseudohalides". Journal of Applied Crystallography. 9 (6): 506–507. Bibcode:1976JApCr...9..506Z. doi:10.1107/S0021889876012041.
1 2 3 Tadeusz Marek Krygowski; Michal Ksawery Cyranski, eds. (2009). Aromaticity in Heterocyclic Compounds. Vol. 19 of Topics in Heterocyclic Chemistry. Springer. pp. 219–220. ISBN 9783540683292.

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] 1 2 Onium compounds, IUPAC Gold Book

[OlahBk-2] 1 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] IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "carbonium ion". doi:10.1351/goldbook.C00839

[5] Yeh, L. I.; Price, J. M.; Lee, Yuan T. (July 1989). "Infrared spectroscopy of the pentacoordinated carbonium ion C2H7+". Journal of the American Chemical Society. 111 (15): 5597–5604. Bibcode:1989JAChS.111.5597Y. doi:10.1021/ja00197a015.

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

[7] Weinert, Charles S. (March 2011). "Synthetic, Structural, and Physical Aspects of Organo-Oligogermanes". Comments on Inorganic Chemistry. 32 (2): 55–87. doi:10.1080/02603594.2011.618854.

[8] Sollott, Gilbert P.; Peterson, William R. (December 1967). "Germylation of ferrocene under Friedel-Crafts conditions. Question of the existence of germonium ions". Journal of the American Chemical Society. 89 (25): 6783–6784. Bibcode:1967JAChS..89.6783S. doi:10.1021/ja01001a082.

[9] Leighton, Kevin L.; Wasylishen, Roderick E. (1 July 1987). "Deuterium isotope effects on the ¹¹⁹Sn shielding constants and spin–spin coupling constants in stannane and the stannonium cation". Canadian Journal of Chemistry. 65 (7): 1469–1473. Bibcode:1987CaJCh..65.1469L. doi:10.1139/v87-250. ISSN 0008-4042.

[smithcaab-10] 1 2 Smith, J. D. (22 October 2013). The Chemistry of Arsenic, Antimony and Bismuth: Pergamon Texts in Inorganic Chemistry. Elsevier. p. 588. ISBN 978-1-4831-8754-9.

[11] Norman, N. C. (31 December 1997). Chemistry of Arsenic, Antimony and Bismuth. Springer Science & Business Media. pp. 310–323. ISBN 978-0-7514-0389-3.

[12] Leicester, Henry M.; Bergstrom, F. W. (1929). "Salts of Triphenylselenonium Hydroxide". Journal of the American Chemical Society. 51 (12): 3587–3591. Bibcode:1929JAChS..51.3587L. doi:10.1021/ja01387a011.

[13] Bainbridge, Kenneth T. (1 July 1933). "Comparison of the Masses of H₂ and Helium". Physical Review. 44 (1): 57. Bibcode:1933PhRv...44...57B. doi:10.1103/PhysRev.44.57.

[14] Kuntz, P. J.; Roach, A. C. (1972). "Ion-molecule reactions of the rare gases with hydrogen. Part 1.—Diatomics-in-molecules potential energy surface for ArH+2". J. Chem. Soc., Faraday Trans. 2. 68: 259–280. doi:10.1039/F29726800259.

[15] Neufeld, David A.; Wolfire, Mark G. (1 August 2016). "The Chemistry of Interstellar Argonium and Other Probes of the Molecular Fraction in Diffuse Clouds". The Astrophysical Journal. 826 (2): 183. arXiv:1607.00375. Bibcode:2016ApJ...826..183N. doi:10.3847/0004-637X/826/2/183.

[16] Linnartz, H.; Zink, L.R.; Evenson, K.M. (July 1997). "The Pure Rotational Spectra of ⁸⁴KrH⁺ and ⁸⁶KrH⁺". Journal of Molecular Spectroscopy. 184 (1): 56–59. Bibcode:1997JMoSp.184...56L. doi:10.1006/jmsp.1997.7297.

[17] Grandinetti, Felice (October 2011). "Gas-Phase Ion Chemistry of the Noble Gases: Recent Advances and Future Perspectives". European Journal of Mass Spectrometry. 17 (5): 423–463. doi:10.1255/ejms.1151.

[18] Mitcham, Renonia V.; Lee, Byungkook; Mertes, Kristin Bowman; Ziolo, Ronald F. (1 December 1979). "The nature of triphenylselenonium chloride. Crystal and molecular structure of the monohydrate: triphenylselenonium chloride hydrate". Inorganic Chemistry. 18 (12): 3498–3502. doi:10.1021/ic50202a041.

[19] Ziolo, R. F.; Titus, D. D. (1 December 1976). "Crystal data for triphenyl telluronium pseudohalides". Journal of Applied Crystallography. 9 (6): 506–507. Bibcode:1976JApCr...9..506Z. doi:10.1107/S0021889876012041.

[krygowski2009-20] 1 2 3 Tadeusz Marek Krygowski; Michal Ksawery Cyranski, eds. (2009). Aromaticity in Heterocyclic Compounds. Vol. 19 of Topics in Heterocyclic Chemistry. Springer. pp. 219–220. ISBN 9783540683292.