Onium ion

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, NH₃.¹²

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.¹²

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

• 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. The specific cation CH+5 is called methanium.⁴. Typically named for the parent hydrocarbon, e.g. C₂H+7 is ethanium.⁵
• silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium⁶)
  • disilanium, Si₂H+7 (protonated disilane)
  • further silanium cations, Si
    _nH⁺
    _2n+3 (protonated silanes)
• germonium, GeH+5 (protonated germane) Unstable derivative known of R₃Ge⁺.⁷⁸
• stannonium, SnH+3 (protonated stannylene, SnH₂) (not protonated stannane SnH₄) stable at cryogenic conditions.⁹

• ammonium (IUPAC name azanium), NH+4 (protonated ammonia (IUPAC name azane)) exists in aqueous solution and as salts.
  • primary, secondary, and tertiary organic derivatives [NH_nR_4−n]⁺, derived from protonation of amines; e.g. methylammonium halides, diethylammonium chloride, trimethylammonium
  • quaternary ammonium cations NR+4; e.g. tetrabutylammonium
• phosphonium, PH+4 (protonated phosphine)
  • primary, secondary, and tertiary organic derivatives [PH_nR_4−n]⁺, derived from protonation of phosphines
  • quaternary phosphonium cations PR+4; e.g. tetraphenylphosphonium
• arsonium, AsH+4 (protonated arsine) known as derivatives or as an unstable salt at cryogenic temperatures.¹⁰
• stibonium, SbH+4 (protonated stibine) known as derivatives but no salts are known.¹⁰
• bismuthonium, BiH+4 (protonated bismuthine) only known as derivatives.¹¹

• 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.)
  • Organic derivatives can be primary (ROH+2, protonated alcohols), secondary (R₂OH⁺, protonated ethers), or ternary (R₃O⁺, as trimethyloxonium).
• peroxonium, H₃O+2 (protonated hydrogen peroxide)
• sulfonium, H₃S⁺ (protonated hydrogen sulfide)
  • Organic derivatives can be primary (H₂SR⁺, protonated thiols), secondary (HSR+2, protonated thioethers), or ternary (SR+3, e.g. trimethylsulfonium)
• selenonium, H₃Se⁺ (protonated hydrogen selenide)
  • Tertiary organic derivatives R₃Se⁺ are known, with trimethylselenonium iodide being the first.¹²
• telluronium, H₃Te⁺ (protonated hydrogen telluride)

• 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)

• 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) known in gas phase.¹³
• neonium, NeH⁺ (protonated neon) known in gas phase.¹⁴
• argonium, ArH⁺ (protonated argon) known in gas phase.¹⁵
• kryptonium, KrH⁺ (protonated krypton) known in gas phase.¹⁶
• xenonium, XeH⁺ (protonated xenon) known in gas phase.¹⁷

• tertiary selenonium cations, R₃Se⁺
  • triphenylselenonium, (C₆H₅)₃Se⁺¹⁸
• tertiary telluronium cations, R₃Te⁺
  • triphenyltelluronium, (C₆H₅)₃Te⁺¹⁹
• 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⁺
• tertiary sulfonium cations having one triple-bonded substitution, R≡S⁺
  • thionitrosyl, N≡S⁺
• dihydroxyoxoammonium, [H₂NO₃]⁺ (protonated nitric acid)
• trihydroxyoxosulfonium, [H₃SO₄]⁺ (protonated sulfuric acid)
• cyclic tertiary onium cations
  • pyrylium, C₅H₅O⁺
  • thiopyrylium, C₅H₅S⁺²⁰
  • selenopyrylium, C₅H₅Se⁺²⁰
  • telluropyrylium, C₅H₅Te⁺²⁰

• 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)