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, , the protonated derivative of ammonia, .

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, . The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium.

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 analogousous to 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 ions when the central atom gains one more bond and becomes a negative anion.

Periodic table

Onium ions by group

Boron groupGroup 13 (boron group) onium cations

boronium cation, (protonated borane)

• further boronium cations, (protonated boranes)

Carbon groupGroup 14 (carbon group) onium cations

• Carbonium ions (protonated hydrocarbons) have a pentacoordinated carbon atom with a +1 charge. The specific cation is called methanium.. Typically named for the parent hydrocarbon, e.g. is ethanium.

silanium (sometimes silonium), (protonated silane) (should not be called siliconium)

• disilanium, (protonated disilane)

• further silanium cations, (protonated silanes)

• germonium, (protonated germane) Unstable derivative known of R3Ge+.

• stannonium, (protonated stannylene, ) (not protonated stannane ) stable at cryogenic conditions.

PnictogenGroup 15 (pnictogen) onium cations

ammonium (IUPAC name azanium), (protonated ammonia (IUPAC name azane)) exists in aqueous solution and as salts.

• primary, secondary, and tertiary organic derivatives , derived from protonation of amines; e.g. methylammonium halides, diethylammonium chloride, trimethylammonium

• quaternary ammonium cations ; e.g. tetrabutylammonium

phosphonium, (protonated phosphine)

• primary, secondary, and tertiary organic derivatives , derived from protonation of phosphines

• quaternary phosphonium cations ; e.g. tetraphenylphosphonium

• arsonium, (protonated arsine) known as derivatives or as an unstable salt at cryogenic temperatures.

• stibonium, (protonated stibine) known as derivatives but no salts are known.

• bismuthonium, (protonated bismuthine) only known as derivatives.

ChalcogenGroup 16 (chalcogen) onium cations

oxonium, (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 (, protonated alcoholss), secondary (, protonated ethers), or ternary (, as trimethyloxonium).

• peroxonium, (protonated hydrogen peroxide)

sulfonium, (protonated hydrogen sulfide)

• Organic derivatives can be primary (, protonated thiols), secondary (, protonated thioethers), or ternary (, e.g. trimethylsulfonium)

selenonium, (protonated hydrogen selenide)

• Tertiary organic derivatives are known, with trimethylselenonium iodide being the first.

• telluronium, (protonated hydrogen telluride)

Hydrogen onium cation

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

HalogenGroup 17 (halogen) onium cations, halonium ionhalonium ions, H2X+ (protonated hydrogen halidehydrogen halides)

• fluoronium, (protonated hydrogen fluoride)

• chloronium, (protonated hydrogen chloride)

• bromonium, (protonated hydrogen bromide)

• iodonium, (protonated hydrogen iodide)

PseudohalogenPseudohalogen onium cations

• aminodiazonium, (protonated hydrogen azide)

• methylidyneammonium and hydrocyanonium, , isomers (protonated hydrogen cyanide)

Noble gasGroup 18 (noble gas) onium cations

• hydrohelium or helonium, better known as helium hydride ion, (protonated helium) known in gas phase.

• neonium, (protonated neon) known in gas phase.

• argonium, (protonated argon) known in gas phase.

• kryptonium, (protonated krypton) known in gas phase.

• xenonium, (protonated xenon) known in gas phase.

Onium cations with monovalent substitutions

tertiary selenonium cations,

• triphenylselenonium,

tertiary telluronium cations,

• triphenyltelluronium,

• primary fluoronium cations, (protonated fluorides RF)

secondary fluoronium cations,

• dichlorofluoronium,

secondary iodonium cations,

• diphenyliodonium,

Onium cations with polyvalent substitutions

secondary ammonium cations having one double-bonded substitution,

• diazenium, (protonated diazene)

• guanidinium, (protonated guanidine) (has a resonance structure and a planar molecular geometry)

tertiary ammonium cations having one triple-bonded substitution, R≡NH+

• nitrilium, (protonated nitrile)

• diazonium or diazynium, (protonated nitrogen, in other words, protonated diazyne)

cyclic tertiary ammonium cations where nitrogen is a member of a ring, (the ring may be aromatic)

• pyridinium, (protonated pyridine)

quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions,

• iminium, (substituted protonated imine)

• diazenium, (substituted protonated diazene)

• thiazolium, (substituted protonated thiazole)

quaternary ammonium cations having two double-bonded substitutions,

• nitronium,

• bis(triphenylphosphine)iminium,

quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution,

• diazonium, (substituted protonated nitrogen, in other words, substituted protonated diazyne)

• nitrilium, (substituted protonated nitrile)

tertiary oxonium cations having one triple-bonded substitution,

• acylium ions,

• nitrosonium,

tertiary sulfonium cations having one triple-bonded substitution,

• thionitrosyl,

• dihydroxyoxoammonium, (protonated nitric acid)

• trihydroxyoxosulfonium, (protonated sulfuric acid)

cyclic tertiary onium cations

• pyrylium,

• thiopyrylium,

• selenopyrylium,

• telluropyrylium,

Double onium dications

• hydrazinediium or hydrazinium(2+) dication, (doubly protonated hydrazine, in other words, doubly protonated diazane)

• diazenediium cation, (doubly protonated diazene)

• diazynediium cation, (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, (protonated borylenes a.k.a. boranylidenes)

carbenium cations, (protonated carbenes) have a tricoordinated carbon atom with a +1 charge.

alkenium cations, (n ≥ 2) (protonated alkenes)

• methenium cation, (protonated methylene)

• ethenium, (protonated ethene)

  benzenium, (protonated benzene)

• tropylium, (protonated tropylidene)

  silylium cations, (protonated silylenes)

• nitrenium cations, (protonated nitrenes)

• phosphinidenium cations, (protonated phosphinidene)

• mercurinium cations, (protonated organomercury compounds; formed as intermediates in oxymercuration reactions)

Substituted eniums

• diphenylcarbenium, (di-substituted methenium)

• triphenylcarbenium, (tri-substituted methenium)

Ynium cations

carbynium ions (protonated carbynes) have a carbon atom with a +1 charge.

alkynium cations, (n ≥ 2) (protonated alkynes)

• methynium cation, (protonated methylidyne radical)

• ethynium, (protonated ethyne)

See also

• Carbonium ion

• Lyonium ion, a protonated solvent molecule

• Lyate ion, a deprotonated solvent molecule

References

External links

• Ions and Radicals, Queen Mary University of London