Onium ion

Not to be confused with onium states such as positronium.

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]

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. ^[4] . Typically named for the parent hydrocarbon, e.g. C₂H+7 is ethanium. ^[5]
• silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium ^[6] )
  • disilanium, Si₂H+7 (protonated disilane)
  • further silanium cations, Si
    _ⁿH⁺
    _2n+3 (protonated silanes)
• germonium, GeH+5 (protonated germane) Unstable derivative known of R₃Ge⁺. ^{[7] [8]}
• stannonium, SnH+3 (protonated stannylene, SnH₂) (not protonated stannane SnH₄) stable at cryogenic conditions. ^[9]

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. ^[10]
• stibonium, SbH+4 (protonated stibine) known as derivatives but no salts are known. ^[10]
• bismuthonium, BiH+4 (protonated bismuthine) only known as derivatives. ^[11]

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).
• 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. ^[12]
• 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. ^[13]
• neonium, NeH⁺ (protonated neon) known in gas phase. ^[14]
• argonium, ArH⁺ (protonated argon) known in gas phase. ^[15]
• kryptonium, KrH⁺ (protonated krypton) known in gas phase. ^[16]
• xenonium, XeH⁺ (protonated xenon) known in gas phase. ^[17]

Onium cations with monovalent substitutions

• tertiary selenonium cations, R₃Se⁺
  • triphenylselenonium, (C₆H₅)₃Se^{+ [18]}
• tertiary telluronium cations, R₃Te⁺
  • triphenyltelluronium, (C₆H₅)₃Te^{+ [19]}
• 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⁺
• 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^{+ [20]}
  • selenopyrylium, C₅H₅Se^{+ [20]}
  • telluropyrylium, C₅H₅Te^{+ [20]}

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)

See also

• Carbonium ion
• Lyonium ion, a protonated solvent molecule
• Lyate ion, a deprotonated solvent molecule

References

1. Onium compounds, IUPAC Gold Book
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. 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. 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. doi:10.1139/v87-250. ISSN   0008-4042.
10. 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 HYDROXIDE¹". Journal of the American Chemical Society. 51 (12): 3587–3591. doi:10.1021/ja01387a011.
13. Bainbridge, Kenneth T. (1 July 1933). "Comparison of the Masses of H₂ and Helium". Physical Review. 44 (1): 57–57. 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 (0): 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. 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. 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. doi:10.1107/S0021889876012041.
20. 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)