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]

Periodic table

Onium ions by group

Group 13 (boron group) onium cations

  • boronium cation, BH+4 (protonated borane)
    • further boronium cations, B
      x      H+
      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
      n      H+
      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.
  • phosphonium, PH+4 (protonated phosphine)
    • primary, secondary, and tertiary organic derivatives [PHnR4−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).
  • 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.[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

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
  • 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+
  • 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

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.

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
      n      H+
      2n-1       (n ≥ 2) (protonated alkynes)
      • methynium cation, H2C+ (protonated methylidyne radical)
      • ethynium, C2H+3 (protonated ethyne)

See also

References

  1. ^ a b Onium compounds, IUPAC Gold Book
  2. ^ a b 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.
  10. ^ a b 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.
  20. ^ a b c 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.
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