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

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

Group 16 (chalcogen) onium cations

Hydrogen onium cation

Group 17 (halogen) onium cations, halonium ions, H2X+ (protonated hydrogen halides)

Pseudohalogen onium cations

Group 18 (noble gas) onium cations

Onium cations with monovalent substitutions

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+
  • cyclic tertiary ammonium cations where nitrogen is a member of a ring, RNH+R (the ring may be aromatic)
  • 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
  • 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.

See also

References

  1. 1 2 Onium compounds, IUPAC Gold Book
  2. 1 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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