Onium ion
In chemistry, an onium compound, or onium ion, is a cation formally obtained by the protonation of mononuclear parent hydride of elements of the nitrogen group (group 15 of the periodic table), chalcogens (group 16), or halogens (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 (nitrogen group/pnictogen) onium cations
- ammonium, (IUPAC name azanium) NH+
4 (protonated ammonia, (IUPAC name azane) - phosphonium, PH+
4 (protonated phosphine) - arsonium, AsH+
4 (protonated arsine) - stibonium, SbH+
4 (protonated stibine) - bismuthonium, BiH+
4 (protonated bismuthine)
Group 16 (chalcogen) onium cations
- oxonium, better known as hydronium, H
3O+
(protonated water) (Hydronium implies a solvated or hydrated proton.) (May be called hydroxonium for disambiguation.) - sulfonium, H
3S+
(protonated hydrogen sulfide) - selenonium, H
3Se+
(protonated hydrogen selenide) - telluronium, H
3Te+
(protonated hydrogen telluride)
Group 17 (halogen) onium cations, halonium ions, H
2X+
(protonated hydrogen halides)
- fluoronium, H
2F+
(protonated hydrogen fluoride) - chloronium, H
2Cl+
(protonated hydrogen chloride) - bromonium, H
2Br+
(protonated hydrogen bromide) - iodonium, H
2I+
(protonated hydrogen iodide)
Pseudohalogen onium cations
- aminodiazonium, H
2N+
3 (protonated hydrogen azide) - hydrocyanonium, H
2CN+
(protonated hydrogen cyanide)
Group 14 (carbon group) onium cations
- methanium, CH+
5 (protonated methane) (Sometimes methonium, but it has multiple definitions.) (Should not be called carbonium[1].) (Abundant in outer space.) - further alkanium cations, CnH(2n+3)+
- ethanium, C
2H+
7 (protonated ethane) - propanium, C
3H+
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
4H+
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)
- n-butanium (n-butane protonated on an unspecified carbon)
- octonium, C
8H+
19 (protonated octane)
- ethanium, C
- silanium (sometimes silonium), SiH+
5 (protonated silane) (Should not be called siliconium.[2]) - further silanium cations, SinH(2n+3)+
- disilanium, Si
2H+
7 (protonated disilane)
- disilanium, Si
- 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
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
- hydrohelium, better known as helium hydride ion, HeH+
(protonated helium). (Abundant in outer space). - kryptonium, KrH+
(protonated krypton) - xenonium, XeH+
(protonated xenon)
Onium cations with monovalent substitutions
- primary ammonium cations, RH
3N+
or RNH+
3 (protonated primary amines)- hydroxylammonium, NH
3OH+
(protonated hydroxylamine) - methylammonium, CH
3NH+
3 (protonated methylamine) - ethylammonium, C
2H
5NH+
3 (protonated ethylamine) - hydrazinium, or diazanium, NH
2NH+
3 (protonated hydrazine, a.k.a. diazane)
- hydroxylammonium, NH
- secondary ammonium cations, R
2NH+
2 (protonated secondary amines)- dimethylammonium (sometimes dimethylaminium), (CH
3)
2NH+
2 (protonated dimethylamine) - diethylammonium (sometimes diethylaminium), (C
2H
5)
2NH+
2 (protonated diethylamine) - ethylmethylammonium, C
2H
5CH
3NH+
2 (protonated ethylmethylamine) - diethanolammonium (sometimes diethanolaminium), (C
2H
4OH)
2NH+
2 (protonated diethanolamine)
- dimethylammonium (sometimes dimethylaminium), (CH
- tertiary ammonium cations, R
3NH+
(protonated tertiary amines)- trimethylammonium (CH
3)
3NH+
(protonated trimethylamine) - triethylammonium (C
2H
5)
3NH+
(protonated triethylamine)
- trimethylammonium (CH
- quaternary ammonium cations, R
4N+
or NR+
4- tetramethylammonium, (CH
3)
4N+ - tetraethylammonium, (C
2H
5)
4N+ - tetrapropylammonium, (C
3H
7)
4N+ - tetrabutylammonium, (C
4H
9)
4N+
or abbreviated Bu
4N+ - trimethyl ammonium compounds, (CH
3)
3RN+ - didecyldimethylammonium, (C
10H
21)
2(CH
3)
2N+ - pentamethylhydrazinium, N(CH
3)
2N(CH
3)+
3
- tetramethylammonium, (CH
- quaternary phosphonium cations, R
4P+
or PR+
4- tetraphenylphosphonium, (C
6H
5)
4P+
- tetraphenylphosphonium, (C
- secondary sulfonium cations, R
2SH+
(protonated sulfides)- dimethylsulfonium, (CH
3)
2SH+
(protonated dimethyl sulfide)
- dimethylsulfonium, (CH
- tertiary sulfonium cations, R
3S+
- trimethylsulfonium, (CH
3)
3S+
- trimethylsulfonium, (CH
- secondary fluoronium cations, R
2F+
- dichlorofluoronium, Cl
2F+
- dichlorofluoronium, Cl
Onium cations with polyvalent substitutions
- secondary ammonium cations having one double-bonded substitution, R=NH2+
- quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR2+
- quaternary ammonium cations having two double-bonded substitutions, R=N=R+
- nitronium, NO+
2 - bis(triphenylphosphine)iminium, ((C
6H
5)
3P)
2N+
- nitronium, NO+
- tertiary ammonium cations having one triple-bonded substitution, R≡NH+
- tertiary ammonium cations having two partially double-bonded substitutions, R=-NH+=-R
- pyridinium, C5H5NH+ (protonated pyridine)
- 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+
- nitrosonium, N≡O+
- tertiary oxonium cations having two partially double-bonded substitutions, R=-O+=-R
- pyrylium, C5H5O+
- tertiary sulfonium cations having one triple-bonded substitution, R≡S+
- thionitrosyl, N≡S+
Double-onium dications
- hydrazinium dication, +H3NNH3+ (doubly protonated hydrazine)
- diazenium dication, +H2N=NH2+ (doubly protonated diazene)
- diazonium dication, +HN≡NH+ (doubly protonated nitrogen)
Unusual onium cations
- Borenium cations, R2B+ (protonated borylenes)
- Carbenium cations, R3C+ (protonated carbenes)
- Silylium cations, R3Si+ (protonated silylenes)
- Nitrenium cations, R2N+ (protonated nitrenes)
- Mercurinium cations, R3Hg+ (protonated organomercury compounds; formed as intermediates in oxymercuration reactions)
External links
- Onium compounds at IUPAC
- Onium compounds at IUPAC
- Overview at chem.qmul.ac.uk
- Onium+compounds at the U.S. National Library of Medicine Medical Subject Headings (MeSH)