Onium compound

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In chemistry, an onium compound, or 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 di- or trivalent, yielding ions such as iminium and nitrilium.[1][2]

Onium ions have a charge of +1. A molecule with two or more onium ion subgroups is said to be a double onium ion, triple onium ion, and so on. Compounds of an onium cation and some other negative ion are known as onium salts.

Onium compounds are inversely analogous to 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, H
2X+
 (protonated hydrogen halides)

Pseudohalogen onium cations

Group 14 (carbon group) onium cations

  • methanium, CH+
    5     (protonated methane) (Sometimes methonium, but it has multiple definitions.) (Should not be called carbonium.) (Abundant in outer space.)
  • further alkanium cations, CnH(2n+3)+
    • 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, C
      8      H+
      19       (protonated octane)
  • silanium (sometimes silonium), SiH+
    5     (protonated silane) (Should not be called siliconium.)
  • further silanium cations, SinH(2n+3)+
    • 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

  • boronium cations, BH+
    3    , BH+
    4    , BH+
    5    , BH+
    6     (protonated boranes)

Hydrogen onium cation

  • hydrogenonium, better known as trihydrogen cation, H+
    3     (protonated [molecular] or [diatomic] hydrogen). (Abundant in outer space).

Group 18 (noble gas) onium cations

Onium cations with monovalent substitutions

  • secondary ammonium cations, R
    2    NH+
    2     (protonated secondary amines)
    • dimethylammonium (sometimes dimethylaminium), (CH
      3      )
      2      NH+
      2       (protonated dimethylamine)
    • diethylammonium (sometimes diethylaminium), (C
      2      H
      5      )
      2      NH+
      2       (protonated diethylamine)
    • ethylmethylammonium, C
      2      H
      5      CH
      3      NH+
      2       (protonated ethylmethylamine)
    • diethanolammonium (sometimes diethanolaminium), (C
      2      H
      4      OH)
      2      NH+
      2       (protonated diethanolamine)
  • tertiary ammonium cations, R
    3    NH+
     (protonated tertiary amines)
  • quaternary phosphonium cations, R
    4    P+
     or PR+
    4
    • tetraphenylphosphonium, (C
      6      H
      5      )
      4      P+
  • secondary fluoronium cations, R
    2    F+
    • dichlorofluoronium, Cl
      2      F+

Onium cations with polyvalent substitutions

  • secondary ammonium cations having one double-bonded substitution, R=NH2+
    • diazenium, HN=NH2+ (protonated diazene)
  • 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)
  • tertiary ammonium cations having one triple-bonded substitution, R≡NH+
  • tertiary ammonium cations having two partially double-bonded substitutions, R=-NH+=-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, R=-O+=-R
  • tertiary sulfonium cations having one triple-bonded substitution, R≡S+
    • thionitrosyl, N≡S+

Double-onium dications

Unusual onium cations

External links

References

  1. ↑ 1.0 1.1 IUPAC definition
  2. ↑ 2.0 2.1 George A. Olah (1998), "Onium Ions". John Wiley & Sons, 509 pages. 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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