Onium ion

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Not to be confused with onium states such as positronium.

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 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 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 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) have 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.[4] 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)
        • , or (propane protonated on an end carbon)
        • (propane protonated on the middle carbon)
      • butanium, C
        4        H+
        11         (butane protonated on an unspecified carbon)
        • (n-butane protonated on an unspecified carbon)
          • , or (n-butane protonated on an end carbon)
          • (n-butane protonated on a middle carbon)
        • isobutanium (isobutane protonated on an unspecified carbon)
          • , or (isobutane protonated on an end carbon)
          • (isobutane protonated on the middle carbon)
      • octonium or octanium, C
        8        H+
        19         (protonated octane)
  • (sometimes ), SiH+
    5     (protonated silane. Should not be called siliconium.[5]
  • further silanium cations, Si
    n    H+
    2n+3     (protonated silanes)
  • , GeH+
    5     (protonated germane)
  • , SnH+
    3     (protonated SnH
    2    ; not protonated stannane SnH
    4    )
  • , PbH+
    3     (protonated PbH
    2    )

Group 13 (boron group) onium cations[]

  • boronium cation, BH+
    4     (protonated borane)
  • further boronium cations, B
    x    H+
    y     (protonated boranes)

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=NH+
    2
    • diazenium, HN=NH+
      2       (protonated diazene)
    • guanidinium, (H
      2      N)
      2      C=NH+
      2       (protonated guanidine) (has a resonance structure)
  • 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 double-bonded substitution and two single-bonded substitutions, R=NR+
    2
    • iminium, R
      2      C=NR+
      2       (substituted protonated imine)
    • diazenium, RN=NR+
      2       (substituted protonated diazene)
    • thiazolium, [C
      3      NSR
      4      ]+
      (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)
    • 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 oxonium cations having two partially double-bonded substitutions, RO+R
  • tertiary sulfonium cations having one triple-bonded substitution, R≡S+
    • , N≡S+

Double onium dications[]

  • hydrazinediium or hydrazinium(2+) dication, +H3NNH+
    3     (doubly protonated hydrazine)
  • cation, +H2N=NH+
    2     (doubly protonated diazene)
  • cation, +HN≡NH+ (doubly protonated [di]nitrogen)

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, (C
    6    H
    5    )
    2    CH+     (di-substituted methenium)
  • triphenylcarbenium, (C
    6    H
    5    )
    3    C+
     (tri-substituted methenium)

Ynium cations[]

  • (protonated carbynes) have a divalent carbon atom with a +1 charge.
    • alkynium cations, C
      n      H+
      2n-1       (n ≥ 2) (protonated alkynes)

See also[]

References[]

  1. ^ Jump up to: a b Onium compounds definition at IUPAC Gold Book
  2. ^ Jump up to: a b 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 definition at IUPAC Gold Book
  5. ^ RC-82. Cations, Queen Mary University of London)

External links[]

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