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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.cite-ref-iupac-1-0[1]cite-ref-olahbk-2-0[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.cite-ref-iupac-1-1[1]cite-ref-olahbk-2-1[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.cite-ref-3[3]

• Simple onium cations (hydrides with no substitutions)

• Group 13 (boron group) onium cations

• Group 14 (carbon group) onium cations

• Group 15 (pnictogen) onium cations

• Group 16 (chalcogen) onium cations

• Hydrogen onium cation

• Group 17 (halogen) onium cations, halonium ions , H 2 X + (protonated hydrogen halides )

• Group 18 (noble gas) onium cations

• Onium cations with monovalent substitutions

• Onium cations with polyvalent substitutions

• Double onium dications

• Enium cations

• Substituted eniums

• Ynium cations

• See also

• References

• External links

• boronium cation, BH+4 (protonated borane)

• further boronium cations, B xH+ y (protonated boranes)

• 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.cite-ref-4[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 siliconiumcite-ref-5[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) (not protonated plumbane PbH4)

• flerovonium, FlH+3 (protonated FlH2) (not protonated flerovane FlH4)

• 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)

• moscovonium, McH+4 (protonated moscovine)

• 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)

• livermoronium, H3Lv+ (protonated hydrogen livermoride)

• hydrogenonium, better known as trihydrogen cation, H+3 (protonated molecular or diatomic hydrogen), found in ionized hydrogen and interstellar space

• 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)

• tennessonium, H2Ts+ (protonated hydrogen tennesside)

• aminodiazonium, [H2N=N=N]+ ⇌ [H2N−N≡N]+ (protonated hydrogen azide)

• methylidyneammonium and hydrocyanonium, H2CN+, isomers HC≡NH+ ⇌ N≡CH+2 (protonated hydrogen cyanide)

• 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)

• oganessonium OgH+ (protonated oganesson)

• 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+

• 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)

• 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)

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)

• diphenylcarbenium, (C6H5)2CH+ (di-substituted methenium)

• triphenylcarbenium, (C6H5)3C+ (tri-substituted methenium)

• 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)

• Carbonium ion

• Lyonium ion, a protonated solvent molecule

• Lyate ion, a deprotonated solvent molecule

cite-note-iupac-11. ↑ Onium compounds, IUPAC Gold Book

cite-note-olahbk-22. ↑ citerefgeorge-a-olah1998George A. Olah (1998). Onium Ions. John Wiley & Sons. p. 509. ISBN 9780471148777.

cite-note-33. ↑ Advanced Organic Chemistry: Reactions and mechanisms, Maya Shankar Singh, 2007, Dorling Kindersley, ISBN 978-81-317-1107-1

cite-note-44. ↑ Carbonium ion, IUPAC Gold Book

cite-note-55. ↑ RC-82. Cations, Queen Mary University of London)

• Ions and Radicals, Queen Mary University of London

• Onium compounds at the U.S. National Library of Medicine Medical Subject Headings (MeSH)