Onium compounds

Onium compounds are cations derived by the protonation of mononuclear parent hydrides of elements of the nitrogen group (Group 15), chalcogens (Group 16), or halogens (Group 17), and similar cations derived by the substitution of hydrogen atoms in the former by other groups, such as organic radicals, or halogens, for example tetramethylammonium, and further derivatives having polyvalent additions, such as iminium and nitrilium. (IUPAC definition [1]; other definitions vary.)

They are also known as onium ions. Compounds of these ions are known as onium salts. Onium ions have a charge of +1; double onium ions have a charge of +2, and so on.

Contents

Simple onium cations

  • Group 14 (carbon group) onium cations (these ions use similar naming schemes. they do not strictly meet the IUPAC definition above):
    • methanium, CH+
      5
      (protonated methane) (Sometimes methonium, but it has multiple definitions.) (Should not be called carbonium[2].) (Abundant in outer space.)
    • further alkanium cations, CnH(2n+3)+
      • ethanium, C2H+
        7
        (protonated ethane)
      • propanium, C3H+
        9
        (propane protonated on an unspecified carbon)
        • propylium, a.k.a. propan-1-ylium (propane protonated on an end carbon)
        • propan-2-ylium (propane protonated on the middle carbon)
    • silanium (sometimes silonium), SiH+
      5
      (protonated silane) (Should not be called siliconium.[3])
    • germonium, GeH+
      5
      (protonated germane)
    • stannonium, SnH+
      3
      (protonated SnH+
      2
      ; not protonated stannane SnH4)
  • Group 13 (boron group) onium cations (these ions use a similar naming scheme. they do not strictly meet the IUPAC definition above):
    • boronium cations, BH+
      3
      , BH+
      4
      , BH+
      5
      , BH+
      6
      (protonated boranes)
  • Hydrogen onium cation (this ion uses a similar naming scheme. it does not strictly meet the IUPAC definition above):
    • hydrogenonium, better known as trihydrogen cation, H+
      3
      (protonated [molecular] or [diatomic] hydrogen). (Abundant in outer space.)

Onium cations with monovalent substitutions

  • secondary ammonium cations, R2NH+
    2
    (protonated secondary amines)
    • dimethylammonium (sometimes dimethylaminium), (CH3)2NH+
      2
      (protonated dimethylamine)
    • diethylammonium (sometimes diethylaminium), (C2H5)2NH+
      2
      (protonated diethylamine)
    • ethylmethylammonium, C2H5CH3NH+
      2
      (protonated ethylmethylamine)
    • diethanolammonium (sometimes diethanolaminium), (C2H4OH)2NH+
      2
      (protonated diethanolamine)
  • tertiary ammonium cations, R3NH+ (protonated tertiary amines)
  • quaternary phosphonium cations, R4P+ or NP+
    4
    • tetraphenylphosphonium, (C6H5)4P+
  • tertiary sulfonium cations, R3S+
    • trimethylsulfonium, (CH3)3S+
  • secondary fluoronium cations, R2F+
    • dichlorofluoronium, Cl2F+

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)
  • quaternary ammonium cations having two double-bonded substitutions, R=N=R+
    • nitronium, NO+
      2
    • bis(triphenylphosphine)iminium, ((C6H5)3P)2N+
  • tertiary ammonium cations having one triple-bonded substitution, R≡NH+
  • quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR+
  • ternary oxonium cations having one triple-bonded substitution, R≡O+
  • tertiary sulfonium cations having one triple-bonded substitution, R≡S+
    • thionitrosyl, N≡S+

Double-onium dications

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