Onium compounds

Onium compounds are cations derived by the protonation of mononuclear parent hydrides of elements of the nitrogen family (Group 15), chalcogen family (Group 16), and halogen family (Group 17), and similar cations derived by the substitution of hydrogen atoms in the former by other groups, such as organic radicals, or halogens, for example tetramethylammonium, and further derivatives having polyvalent additions, such as iminium and nitrilium. (IUPAC definition; other definitions vary.)

They are also known as onium ions. Compounds of these ions are known as onium salts. Onium ions have a charge of +1; double onium ions have a charge of +2, and so on.

Simple onium compounds

 * ammonium, NH4+ (protonated ammonia)
 * phosphonium, PH4+ (protonated phosphine)
 * arsonium, AsH4+ (protonated arsine)
 * stibonium, SbH4+ (protonated stibine)
 * bismuthonium, BiH4+ (protonated bismuthine)
 * oxonium, H3O+, (protonated water) More commonly called by its older name hydronium, which implies a solvated or hydrated proton. May be called hydroxonium for disambiguation.
 * sulfonium, H3S+ (protonated hydrogen sulfide)
 * selenonium, H3Se+ (protonated hydrogen selenide)
 * telluronium, H3Te+ (protonated hydrogen telluride)
 * halonium ions (protonated hydrogen halides)
 * fluoronium, H2F+ (protonated hydrogen fluoride)
 * chloronium, H2Cl+ (protonated hydrogen chloride)
 * bromonium, H2Br+ (protonated hydrogen bromide)
 * iodonium, H2I+ (protonated hydrogen iodide)

The following ions use a similar naming scheme, but they do not strictly meet the above definition. They are protonated hydrides of Carbon family (Group 14) elements:
 * methanium, CH5+ (protonated methane) (Should not be called carbonium or methonium.) (Abundant in outer space.)
 * silanium, SiH5+ (protonated silane) (Should not be called siliconium.)

The following ions use a similar naming scheme, but they do not strictly meet the above definition. They are protonated hydrides of Boron family (Group 13) elements:
 * boronium cations, BH3+, BH4+, BH5+, BH6+ (protonated boron hydrides)

The following ion uses a similar naming scheme, but it does not strictly meet the above definition. It is protonated diatomic hydrogen:
 * hydrogenonium, H3+ (protonated hydrogen) Also known as trihydrogen cation or protonated molecular hydrogen. (Abundant in outer space.)

Onium compounds with monovalent substitutions

 * primary ammonium cations, RH3N+ or RNH3+ (protonated primary amines)
 * hydroxylammonium, NH3OH+ (protonated hydroxylamine)
 * ethylammonium, C2H5NH3+ (protonated ethylamine)
 * hydrazinium, NH2NH3+ (protonated hydrazine)


 * secondary ammonium cations, R2NH2+ (protonated secondary amines)
 * dimethylammonium (sometimes dimethylaminium), (CH3)2NH2+ (protonated dimethylamine)
 * diethanolammonium (sometimes diethanolaminium), (C2H4OH)2NH2+ (protonated diethanolamine)


 * tertiary ammonium cations, R3NH+ (protonated tertiary amines)
 * trimethylammonium (CH3)3NH+ (protonated trimethylamine)


 * quaternary ammonium cations, R4N+ or NR4+
 * tetramethylammonium, (CH3)4N+
 * tetraethylammonium, (CH3CH2)4N+
 * tetrabutylammonium, (C4H9)4N+ or abbreviated Bu4N+
 * trimethyl ammonium compounds, (CH3)3RN+
 * didecyldimethylammonium, (C10H21)2(CH3)2N+
 * pentamethylhydrazinium, N(CH3)2N(CH3)3+


 * quaternary phosphonium cations, R4P+ or NP4+
 * tetraphenylphosphonium, (C6H5)4P+


 * secondary sulfonium cations, R2SH+ (protonated sulfides)
 * dimethylsulfonium, (CH3)2SH+ (protonated dimethyl sulfide)


 * tertiary sulfonium cations, R3S+
 * trimethylsulfonium, (CH3)3S+


 * secondary fluoronium cations, R2F+
 * dichlorofluoronium, Cl2F+


 * various methanium cations
 * ethanium, C2H7+ (protonated ethane)
 * propanium, C3H11+ (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)

Onium compounds with polyvalent substitutions

 * quaternary ammonium cations having one double-bonded substitution, R=NH2+
 * diazenium, HN=NH2+ (protonated diazene)


 * quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR2+
 * iminium, R2C=NR2+ (substituted protonated imine)
 * ??, RN=NR2+


 * 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, R≡NH+
 * nitrilium, RC≡NH+ (protonated nitrile)


 * quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR+
 * diazonium, N≡NR+ (substituted protonated nitrogen)
 * nitrilium?, RC≡NR+ (substituted protonated nitrile)


 * ternary oxonium cations having one triple-bonded substitution, R≡O+
 * nitrosonium, N≡O+


 * tertiary sulfonium cations having one triple-bonded substitution, R≡S+
 * thionitrosyl, N≡S+

Double onium compounds

 * hydrazinium dication, (NH3)2+2 (doubly-protonated hydrazine)
 * diazenium dication, (NH2)2+2 (doubly-protonated diazene)
 * diazonium dication, (NH)2+2 (doubly-protonated nitrogen)