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.[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]

Simple onium cations (hydrides with no substitutions)

Group 13 (boron group) onium cations
  • boronium cation, BH+4 (protonated borane)
    • further boronium cations, B
      x      H+
      y       (protonated boranes)

Group 14 (carbon group) onium cations
  • carbonium ions (protonated hydrocarbons) have a pentacoordinated carbon atom with a +1 charge.
    • alkanium cations, C
      n      H+
      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)
      • octonium or octanium, C8H+19 (protonated octane)
  • silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium[5]
    • disilanium, Si2H+7 (protonated disilane)
    • further silanium cations, Si
      n      H+
      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)

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)

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

Onium cations with monovalent substitutions
  • 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
  • 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+

Onium cations with polyvalent substitutions
  • secondary ammonium cations having one double-bonded substitution, R=NH+2
  • 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+
  • cyclic tertiary oxonium cations where oxygen is a member of a ring, RO+R (the ring may be aromatic)
  • tertiary sulfonium cations having one triple-bonded substitution, R≡S+
    • thionitrosyl, N≡S+
  • dihydroxyoxoammonium, [H2NO3]+ (protonated nitric acid)
  • trihydroxyoxosulfonium, [H3SO4]+ (protonated sulfuric acid)

Double onium dications
  • 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)

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.

Substituted eniums
  • diphenylcarbenium, (C6H5)2CH+ (di-substituted methenium)
  • triphenylcarbenium, (C6H5)3C+ (tri-substituted methenium)

Ynium cations
  • carbynium ions (protonated carbynes) have a carbon atom with a +1 charge.
    • alkynium cations, C
      n      H+
      2n-1       (n ≥ 2) (protonated alkynes)
      • methynium cation, H2C+ (protonated methylidyne radical)
      • ethynium, C2H+3 (protonated ethyne)

See also

References
  1. Onium compounds, IUPAC Gold Book
  2. George A. Olah (1998). Onium Ions. John Wiley & Sons. p. 509. ISBN 9780471148777.
  3. Advanced Organic Chemistry: Reactions and mechanisms, Maya Shankar Singh, 2007, Dorling Kindersley, ISBN 978-81-317-1107-1
  4. Carbonium ion, IUPAC Gold Book
  5. RC-82. Cations, Queen Mary University of London)
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