Onium ion

{{Short description|Class of positively-charged molecules}} {{Distinguish|text=onium states such as positronium}} {{Set index article}} {{more citations needed|date=February 2026}}

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, {{chem2|NH4+}}, the protonated derivative of ammonia, {{chem2|NH3}}.<ref name=IUPAC/><ref name=OlahBk/>

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, {{chem2|(C6H5)4P+}}. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium.<ref name=IUPAC/><ref name=OlahBk/>

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 {{nowrap|''-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 {{nowrap|''-ate''}} ions when the central atom gains one more bond and becomes a negative anion.<ref>''Advanced Organic Chemistry: Reactions and mechanisms'', Maya Shankar Singh, 2007, Dorling Kindersley, {{ISBN|978-81-317-1107-1}}</ref>

== Periodic table == {{Binary polyatomic hydrogen ions}}

== Onium ions by group ==

=== Group 13 (boron group) onium cations === *boronium cation, {{chem2|BH4+}} (protonated borane) **further boronium cations, {{chem|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 {{chem2|CH5+}} is called methanium.<ref>{{GoldBookRef|file=C00839|title=carbonium ion}}</ref>. Typically named for the parent hydrocarbon, e.g. {{chem2|C2H7+}} is ethanium.<ref>{{cite journal |last1=Yeh |first1=L. I. |last2=Price |first2=J. M. |last3=Lee |first3=Yuan T. |title=Infrared spectroscopy of the pentacoordinated carbonium ion C2H7+ |journal=Journal of the American Chemical Society |date=July 1989 |volume=111 |issue=15 |pages=5597–5604 |doi=10.1021/ja00197a015 |bibcode=1989JAChS.111.5597Y }}</ref> *silanium (sometimes silonium), {{chem2|SiH5+}} (protonated silane) (should not be called siliconium<ref>[https://iupac.qmul.ac.uk/ions/RC821.html RC-82. Cations], Queen Mary University of London)</ref>) **disilanium, {{chem2|Si2H7+}} (protonated disilane) **further silanium cations, {{chem|Si|''n''|H|2''n''+3|+}} (protonated silanes) *germonium, {{chem2|GeH5+}} (protonated germane) Unstable derivative known of R3Ge+.<ref>{{cite journal |last1=Weinert |first1=Charles S. |title=Synthetic, Structural, and Physical Aspects of Organo-Oligogermanes |journal=Comments on Inorganic Chemistry |date=March 2011 |volume=32 |issue=2 |pages=55–87 |doi=10.1080/02603594.2011.618854}}</ref><ref>{{cite journal |last1=Sollott |first1=Gilbert P. |last2=Peterson |first2=William R. |title=Germylation of ferrocene under Friedel-Crafts conditions. Question of the existence of germonium ions |journal=Journal of the American Chemical Society |date=December 1967 |volume=89 |issue=25 |pages=6783–6784 |doi=10.1021/ja01001a082 |bibcode=1967JAChS..89.6783S }}</ref> *stannonium, {{chem2|SnH3+}} (protonated stannylene, {{chem2|SnH2}}) (not protonated stannane {{chem2|SnH4}}) stable at cryogenic conditions.<ref>{{cite journal |last1=Leighton |first1=Kevin L. |last2=Wasylishen |first2=Roderick E. |title=Deuterium isotope effects on the 119Sn shielding constants and spin–spin coupling constants in stannane and the stannonium cation |journal=Canadian Journal of Chemistry |date=1 July 1987 |volume=65 |issue=7 |pages=1469–1473 |doi=10.1139/v87-250 |bibcode=1987CaJCh..65.1469L |language=en |issn=0008-4042}}</ref>

===Group 15 (pnictogen) onium cations=== *ammonium (IUPAC name azanium), {{chem2|NH4+}} (protonated ammonia (IUPAC name azane)) exists in aqueous solution and as salts. ** primary, secondary, and tertiary organic derivatives {{chem2|[NH_{n}R_{4-n}]+}}, derived from protonation of amines; e.g. methylammonium halides, diethylammonium chloride, trimethylammonium ** quaternary ammonium cations {{chem2|NR4+}}; e.g. tetrabutylammonium *phosphonium, {{chem2|PH4+}} (protonated phosphine) ** primary, secondary, and tertiary organic derivatives {{chem2|[PH_{n}R_{4-n}]+}}, derived from protonation of phosphines ** quaternary phosphonium cations {{chem2|PR4+}}; e.g. tetraphenylphosphonium *arsonium, {{chem2|AsH4+}} (protonated arsine) known as derivatives or as an unstable salt at cryogenic temperatures.<ref name=smithcaab/> *stibonium, {{chem2|SbH4+}} (protonated stibine) known as derivatives but no salts are known.<ref name=smithcaab>{{cite book |last1=Smith |first1=J. D. |title=The Chemistry of Arsenic, Antimony and Bismuth: Pergamon Texts in Inorganic Chemistry |date=22 October 2013 |publisher=Elsevier |isbn=978-1-4831-8754-9 |page=588 |url=https://books.google.com/books?id=qkP-BAAAQBAJ&q=stibonium+&pg=PP1 |language=en}}</ref> *bismuthonium, {{chem2|BiH4+}} (protonated bismuthine) only known as derivatives.<ref>{{cite book |last1=Norman |first1=N. C. |title=Chemistry of Arsenic, Antimony and Bismuth |date=31 December 1997 |publisher=Springer Science & Business Media |isbn=978-0-7514-0389-3 |pages=310–323 |url=https://books.google.com/books?id=vVhpurkfeN4C&q=bismuthonium&pg=PA283 |language=en}}</ref>

===Group 16 (chalcogen) onium cations=== *oxonium, {{chem2|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 ({{chem2|ROH2+}}, protonated alcohols), secondary ({{chem2|R2OH+}}, protonated ethers), or ternary ({{chem2|R3O+}}, as trimethyloxonium). *peroxonium, {{chem2|H3O2+}} (protonated hydrogen peroxide) *sulfonium, {{chem2|H3S+}} (protonated hydrogen sulfide) ** Organic derivatives can be primary ({{chem2|H2SR+}}, protonated thiols), secondary ({{chem2|HSR2+}}, protonated thioethers), or ternary ({{chem2|SR3+}}, e.g. trimethylsulfonium) *selenonium, {{chem2|H3Se+}} (protonated hydrogen selenide) ** Tertiary organic derivatives {{chem2|R3Se+}} are known, with trimethylselenonium iodide being the first.<ref>{{cite journal|last1=Leicester |first1=Henry M. |last2=Bergstrom |first2=F. W. |title= Salts of Triphenylselenonium Hydroxide |journal=Journal of the American Chemical Society |date=1929 |volume=51 |issue=12 |pages=3587–3591 |doi=10.1021/ja01387a011 |bibcode=1929JAChS..51.3587L }}</ref> *telluronium, {{chem2|H3Te+}} (protonated hydrogen telluride)

===Hydrogen onium cation=== *hydrogenonium, better known as trihydrogen cation, {{chem2|H3+}} (protonated molecular or diatomic hydrogen), found in ionized hydrogen and interstellar space

===Group 17 (halogen) onium cations, halonium ions, {{chem2|H2X+}} (protonated hydrogen halides)=== *fluoronium, {{chem2|H2F+}} (protonated hydrogen fluoride) *chloronium, {{chem2|H2Cl+}} (protonated hydrogen chloride) *bromonium, {{chem2|H2Br+}} (protonated hydrogen bromide) *iodonium, {{chem2|H2I+}} (protonated hydrogen iodide)

====Pseudohalogen onium cations==== *aminodiazonium, {{chem2|[H2N\dN\dN]+ ⇌ [H2N\sN\tN]+}} (protonated hydrogen azide) *methylidyneammonium and hydrocyanonium, {{chem2|H2CN+}}, isomers {{chem2|HC\tNH+ ⇌ N\tCH2+}} (protonated hydrogen cyanide)

===Group 18 (noble gas) onium cations=== *hydrohelium or helonium, better known as helium hydride ion, {{chem2|HeH+}} (protonated helium) known in gas phase.<ref>{{cite journal |last1=Bainbridge |first1=Kenneth T. |title=Comparison of the Masses of H2 and Helium |journal=Physical Review |date=1 July 1933 |volume=44 |issue=1 |pages=57 |doi=10.1103/PhysRev.44.57 |bibcode=1933PhRv...44...57B }}</ref> *neonium, {{chem2|NeH+}} (protonated neon) known in gas phase.<ref>{{cite journal |last1=Kuntz |first1=P. J. |last2=Roach |first2=A. C. |title=Ion-molecule reactions of the rare gases with hydrogen. Part 1.—Diatomics-in-molecules potential energy surface for ArH+2 |journal=J. Chem. Soc., Faraday Trans. 2 |date=1972 |volume=68 |pages=259–280 |doi=10.1039/F29726800259}}</ref> *argonium, {{chem2|ArH+}} (protonated argon) known in gas phase.<ref>{{cite journal |last1=Neufeld |first1=David A. |last2=Wolfire |first2=Mark G. |title=The Chemistry of Interstellar Argonium and Other Probes of the Molecular Fraction in Diffuse Clouds |journal=The Astrophysical Journal |date=1 August 2016 |volume=826 |issue=2 |pages=183 |doi=10.3847/0004-637X/826/2/183 |doi-access=free |arxiv=1607.00375 |bibcode=2016ApJ...826..183N }}</ref> *kryptonium, {{chem2|KrH+}} (protonated krypton) known in gas phase.<ref>{{cite journal |last1=Linnartz |first1=H. |last2=Zink |first2=L.R. |last3=Evenson |first3=K.M. |title=The Pure Rotational Spectra of 84KrH+ and 86KrH+ |journal=Journal of Molecular Spectroscopy |date=July 1997 |volume=184 |issue=1 |pages=56–59 |doi=10.1006/jmsp.1997.7297 |bibcode=1997JMoSp.184...56L }}</ref> *xenonium, {{chem2|XeH+}} (protonated xenon) known in gas phase.<ref>{{cite journal |last1=Grandinetti |first1=Felice |title=Gas-Phase Ion Chemistry of the Noble Gases: Recent Advances and Future Perspectives |journal=European Journal of Mass Spectrometry |date=October 2011 |volume=17 |issue=5 |pages=423–463 |doi=10.1255/ejms.1151}}</ref>

==Onium cations with monovalent substitutions== *tertiary selenonium cations, {{chem2|R3Se+}} **triphenylselenonium, {{chem2|(C6H5)3Se+}}<ref>{{cite journal |last1=Mitcham |first1=Renonia V. |last2=Lee |first2=Byungkook |last3=Mertes |first3=Kristin Bowman |last4=Ziolo |first4=Ronald F. |title=The nature of triphenylselenonium chloride. Crystal and molecular structure of the monohydrate: triphenylselenonium chloride hydrate |journal=Inorganic Chemistry |date=1 December 1979 |volume=18 |issue=12 |pages=3498–3502 |doi=10.1021/ic50202a041}}</ref> *tertiary telluronium cations, {{chem2|R3Te+}} **triphenyltelluronium, {{chem2|(C6H5)3Te+}}<ref>{{cite journal |last1=Ziolo |first1=R. F. |last2=Titus |first2=D. D. |title=Crystal data for triphenyl telluronium pseudohalides |journal=Journal of Applied Crystallography |date=1 December 1976 |volume=9 |issue=6 |pages=506–507 |doi=10.1107/S0021889876012041 |bibcode=1976JApCr...9..506Z }}</ref> *primary fluoronium cations, {{chem2|RFH+}} (protonated fluorides RF) *secondary fluoronium cations, {{chem2|R2F+}} **dichlorofluoronium, {{chem2|Cl2F+}} *secondary iodonium cations, {{chem2|R2I+}} **diphenyliodonium, {{chem2|(C6H5)2I+}}

==Onium cations with polyvalent substitutions== *secondary ammonium cations having one double-bonded substitution, {{chem2|R\dNH2+}} **diazenium, {{chem2|HN\dNH2+}} (protonated diazene) **guanidinium, {{chem2|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, {{chem2|R\sC\tNH+}} (protonated nitrile) **diazonium or diazynium, {{chem2|N\tNH+}} (protonated nitrogen, in other words, protonated diazyne) *cyclic tertiary ammonium cations where nitrogen is a member of a ring, {{chem2|RNH+R}} (the ring may be aromatic) **pyridinium, {{chem2|C5H5NH+}} (protonated pyridine) *quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, {{chem2|R\dNR2+}} **iminium, {{chem2|R2C\dNR2+}} (substituted protonated imine) **diazenium, {{chem2|RN\dNR2+}} (substituted protonated diazene) **thiazolium, {{chem2|[C3NSR4]+}}(substituted protonated thiazole) *quaternary ammonium cations having two double-bonded substitutions, {{chem2|R\dN+\dR}} **nitronium, {{chem2|[NO2]+}} **bis(triphenylphosphine)iminium, {{chem2|((C6H5)3P\d)2N+}} *quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, {{chem2|R\tNR+}} **diazonium, {{chem2|N\tNR+}} (substituted protonated nitrogen, in other words, substituted protonated diazyne) **nitrilium, {{chem2|RC\tNR+}} (substituted protonated nitrile) *tertiary oxonium cations having one triple-bonded substitution, {{chem2|R\tO+}} **acylium ions, {{chem2|R\sC\tO+ ↔ R\sC+\dO}} **nitrosonium, {{chem2|N\tO+}} *tertiary sulfonium cations having one triple-bonded substitution, {{chem2|R\tS+}} **thionitrosyl, {{chem2|N\tS+}} *dihydroxyoxoammonium, {{chem2|[H2NO3]+}} (protonated nitric acid) *trihydroxyoxosulfonium, {{chem2|[H3SO4]+}} (protonated sulfuric acid) *cyclic tertiary onium cations **pyrylium, {{chem2|C5H5O+}} **thiopyrylium, {{chem2|C5H5S+}}<ref name="krygowski2009">{{cite book | title = Aromaticity in Heterocyclic Compounds | volume = 19 of Topics in Heterocyclic Chemistry |editor1= Tadeusz Marek Krygowski |editor2=Michal Ksawery Cyranski | publisher = Springer | year = 2009 | isbn = 9783540683292 | pages = 219–220}}</ref> **selenopyrylium, {{chem2|C5H5Se+}}<ref name="krygowski2009"/> **telluropyrylium, {{chem2|C5H5Te+}}<ref name="krygowski2009"/>

==Double onium dications== *hydrazinediium or hydrazinium(2+) dication, {{chem2|H3N+\s+NH3}} (doubly protonated hydrazine, in other words, doubly protonated diazane) *diazenediium cation, {{chem2|H2N+\d+NH2}} (doubly protonated diazene) *diazynediium cation, {{chem2|HN+\t+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, {{chem2|R2B+}} (protonated borylenes a.k.a. boranylidenes) *carbenium cations, {{chem2|R3C+}} (protonated carbenes) have a tricoordinated carbon atom with a +1 charge. **alkenium cations, {{chem|C|''n''|H|2''n''+1|+}} (''n'' ≥ 2) (protonated alkenes) ***methenium cation, {{chem2|H3C+}} (protonated methylene) ***ethenium, {{chem2|C2H5+}} (protonated ethene) **benzenium, {{chem2|C6H7+}} (protonated benzene) **tropylium, {{chem2|C7H7+}} (protonated tropylidene) *silylium cations, {{chem2|R3Si+}} (protonated silylenes) *nitrenium cations, {{chem2|R2N+}} (protonated nitrenes) *phosphinidenium cations, {{chem2|R2P+}} (protonated phosphinidene) *mercurinium cations, {{chem2|R3Hg+}} (protonated organomercury compounds; formed as intermediates in oxymercuration reactions)

===Substituted eniums=== *diphenylcarbenium, {{chem2|(C6H5)2CH+}} (di-substituted methenium) *triphenylcarbenium, {{chem2|(C6H5)3C+}} (tri-substituted methenium)

==Ynium cations== *carbynium ions (protonated carbynes) have a carbon atom with a +1 charge. **alkynium cations, {{chem|C|''n''|H|2''n''-1|+}} (''n'' ≥ 2) (protonated alkynes) ***methynium cation, {{chem2|H2C+}} (protonated methylidyne radical) ***ethynium, {{chem2|C2H3+}} (protonated ethyne)

==See also== *Carbonium ion *Lyonium ion, a protonated solvent molecule *Lyate ion, a deprotonated solvent molecule

==References== {{Reflist|refs= <ref name=IUPAC>[https://goldbook.iupac.org/terms/view/O04291 Onium compounds], IUPAC Gold Book</ref> <ref name=OlahBk>{{cite book |author=George A. Olah |year=1998 |title=Onium Ions |publisher=John Wiley & Sons |pages=509 |isbn=9780471148777}}</ref> }}

==External links== *[https://iupac.qmul.ac.uk/class/ionra.html#36 Ions and Radicals], Queen Mary University of London *{{MeSH name|Onium compounds}}

Category:Cations Category:Chemical nomenclature