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]

Not to be confused with onium states such as positronium.

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)

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. 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)
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.