Onium ion – Wikipedia

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Class of positively-charged molecules

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 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:

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  • 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)[edit]

Group 13 (boron group) onium cations[edit]

Group 14 (carbon group) onium cations[edit]

  • 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)
      • butanium, C4H+11 (butane protonated on an unspecified carbon)
      • octonium or octanium, C8H+19 (protonated octane)
  • silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium[5]
  • germonium, GeH+5 (protonated germane)
  • stannonium, SnH+3 (protonated SnH2) (not protonated stannane SnH4)
  • plumbonium, PbH+3 (protonated PbH2)

Group 15 (pnictogen) onium cations[edit]

Group 16 (chalcogen) onium cations[edit]

Hydrogen onium cation[edit]

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

Pseudohalogen onium cations[edit]

Group 18 (noble gas) onium cations[edit]

Onium cations with monovalent substitutions[edit]

  • primary ammonium cations, RH3N+ or R−NH+3 (protonated primary amines)
  • secondary ammonium cations, R2NH+2 (protonated secondary amines)
  • tertiary ammonium cations, R3NH+ (protonated tertiary amines)
  • quaternary ammonium cations, R4N+ or NR+4
  • quaternary phosphonium cations, R4P+ or PR+4
  • quaternary arsonium cations, R4As+ or AsR+4
  • quaternary stibonium cations, R4Sb+ or SbR+4
  • primary oxonium cations, ROH+2 (protonated alcohols R−O−H)
    • alkyloxonium cations ROH+2 (protonated alcohols)
      • methyloxonium, CH3OH+2 (protonated methanol)
      • ethyloxonium, CH3CH2OH+2 (protonated ethanol)
  • secondary oxonium cations, R2OH+ (protonated ethers R−O−R)
    • dialkyloxonium cations (protonated ethers)
  • tertiary oxonium cations, R3O+
  • primary sulfonium cations, RSH+2 (protonated thiols R−S−H)
  • secondary sulfonium cations, R2SH+ (protonated thioethers R−S−R)
  • tertiary sulfonium cations, R3S+
  • tertiary selenonium cations, R3Se+
  • tertiary telluronium cations, R3Te+
  • primary fluoronium cations, RFH+ (protonated fluorides RF)
  • secondary fluoronium cations, R2F+
  • secondary iodonium cations, R2I+

Onium cations with polyvalent substitutions[edit]

  • 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+

Double onium dications[edit]

Enium cations[edit]

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[edit]

  • diphenylcarbenium, (C6H5)2CH+ (di-substituted methenium)
  • triphenylcarbenium, (C6H5)3C+ (tri-substituted methenium)

Ynium cations[edit]

See also[edit]

References[edit]

External links[edit]



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