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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.[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]
Group 13 (boron group) onium cations
Group 14 (carbon group) onium cations
- 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.[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)
- n-butanium (n-butane protonated on an unspecified carbon)
- octonium or octanium, C8H+19 (protonated octane)
- alkanium cations, C
- silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium[5]
- 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)
Group 15 (pnictogen) onium cations
- 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)
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.)
- 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)
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)
- astatonium, H2At+ (protonated hydrogen astatide)
- tennessonium, H2Ts+ (protonated hydrogen tennesside)
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)
- 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)
- 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)
- dialkyloxonium cations (protonated ethers)
- 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+
- 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
- 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.
- 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)
- 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)
- alkynium cations, C
- Carbonium ion
- Lyonium ion, a protonated solvent molecule
- Lyate ion, a deprotonated solvent molecule
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