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, 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 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 negative ion 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 15 (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, H
3O+
(protonated water (IUPAC name oxidane). Oxonium is better known as hydronium, though hydronium implies a solvated or hydrated proton. It may also be called hydroxonium.) - 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, H2X+ (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
- carbonium ions, protonated hydrocarbons, having a pentavalent carbon atom with a +1 charge
- alkanium cations, C
nH+
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 [1]. Sometimes called methonium, but methonium also has multiple definitions. Abundant in outer space.) - 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 or octanium, C
8H+
19 (protonated octane)
- methanium, CH+
- alkenium cations, C
nH+
2n+1 (n >= 2) (protonated alkenes) - alkynium cations, C
nH+
2n-1 (n >= 2) (protonated alkynes)
- alkanium cations, C
- silanium (sometimes silonium), SiH+
5 (protonated silane. Should not be called siliconium.[2]) - further silanium cations, Si
nH+
2n+3 (protonated silanes)- 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
Group 18 (noble gas) onium cations
- hydrohelium or helonium, better known as helium hydride ion, HeH+
(protonated helium), expected in outer space, but not yet detected - neonium, NeH+
(protonated neon) - argonium, ArH+
(protonated argon) - kryptonium, KrH+
(protonated krypton) - xenonium, XeH+
(protonated xenon)
Hydrogen onium cation
- hydrogenonium, better known as trihydrogen cation, H+
3 (protonated [molecular] or [diatomic] hydrogen), found in ionized hydrogen and interstellar space
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 - tetrafluoroammonium, NF+
4
- 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
- trifluoroxenonium, XeF+
3 (XeF
2 is neutral.)
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, RNH+R
- pyridinium, C
5H
5NH+
(protonated pyridine)
- pyridinium, C
- 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, RO+R
- pyrylium, C
5H
5O+
- pyrylium, C
- tertiary sulfonium cations having one triple-bonded substitution, R≡S+
- thionitrosyl, N≡S+
Double onium dications
- hydrazinediium or hydrazinium(2+) dication, +
H
3NNH+
3 (doubly protonated hydrazine) - diazenium dication, +
H
2N=NH+
2 (doubly protonated diazene) - diazonium dication, +
HN≡NH+
(doubly protonated [di]nitrogen)
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.
- borenium cations, R
2B+
(protonated borylenes a.k.a. boranylidenes) - carbenium cations, R
3C+
(protonated carbenes, have a trivalent carbon atom with a +1 charge) - silylium cations, R
3Si+
(protonated silylenes) - nitrenium cations, R
2N+
(protonated nitrenes) - phosphinidenium cations, R
2P+
(protonated phosphinidene) - mercurinium cations, R
3Hg+
(protonated organomercury compounds; formed as intermediates in oxymercuration reactions)
Ynium cations
- carbynium ions, protonated carbynes, have a divalent carbon atom with a +1 charge
- methynium cation, H
2C+
(protonated methylidyne radical)
- methynium cation, H
External links
- Overview at chem.qmul.ac.uk
- Onium compounds at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
References
- ^ a b Onium compounds definition at IUPAC Gold Book
- ^ a b George A. Olah (1998). Onium Ions. John Wiley & Sons. p. 509. ISBN 9780471148777.
- ^ Advanced Organic Chemistry: Reactions and mechanisms, Maya Shankar Singh, 2007, Dorling Kindersley, ISBN 978-81-317-1107-1