Onium compound

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, NH
3.[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 radicals, or halogens; such as tetraphenylphosphonium, (C
6H
5)
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 negative ion 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 15 (pnictogen) onium cations

Group 16 (chalcogen) onium cations

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

Pseudohalogen onium cations

Group 14 (carbon group) onium cations

  • carbonium ions, protonated hydrocarbons, having a pentavalent 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 . Sometimes called methonium, but methonium also has multiple definitions. Abundant in outer space.)
      • ethanium, C
        2        H+
        7         (protonated ethane)
      • propanium, C
        3        H+
        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, C
        4        H+
        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, C
        8        H+
        19         (protonated octane)
    • alkenium cations, C
      n      H+
      2n+1       (n >= 2) (protonated alkenes)
    • alkynium cations, C
      n      H+
      2n-1       (n >= 2) (protonated alkynes)
      • ethynium, C
        2        H+
        3         (protonated ethyne)
  • silanium (sometimes silonium), SiH+
    5     (protonated silane. Should not be called siliconium.)
  • further silanium cations, Si
    n    H+
    2n+3     (protonated silanes)
    • disilanium, Si
      2      H+
      7       (protonated disilane)
  • germonium, GeH+
    5     (protonated germane)
  • stannonium, SnH+
    3     (protonated SnH
    2    ; not protonated stannane SnH
    4    )
  • plumbonium, PbH+
    3     (protonated PbH
    2    )

Group 13 (boron group) onium cations

Group 18 (noble gas) onium cations

Hydrogen onium cation

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

Onium cations with monovalent substitutions

Onium cations with polyvalent substitutions

  • secondary ammonium cations having one double-bonded substitution, R=NH2+
  • quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR2+
    • iminium, R2C=NR2+ (substituted protonated imine)
    • diazenium, RN=NR2+ (substituted protonated diazene)
  • quaternary ammonium cations having two double-bonded substitutions, R=N=R+
  • tertiary ammonium cations having one triple-bonded substitution, R≡NH+
  • tertiary ammonium cations having two partially double-bonded substitutions, RNH+R
  • quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR+
  • tertiary oxonium cations having one triple-bonded substitution, R≡O+
  • tertiary oxonium cations having two partially double-bonded substitutions, RO+R
  • tertiary sulfonium cations having one triple-bonded substitution, R≡S+
    • thionitrosyl, N≡S+

Double onium dications

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.

Ynium cations

  • carbynium ions, protonated carbynes, have a divalent carbon atom with a +1 charge

References

  1. 1 2 Onium compounds definition at 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
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