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]
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, (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 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) - HCNH+/methylidyneammonium/hydrocyanonium, H
2CN+
HC≡NH+ (isomer N≡CH+
2) (protonated hydrogen cyanide)
Group 14 (carbon group) onium cations
- carbonium ions (protonated hydrocarbons) have 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.[4] 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+
- alkanium cations, C
- silanium (sometimes silonium), SiH+
5 (protonated silane. Should not be called siliconium.[5] - 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)
- 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, RH3N+ or RNH+
3 (protonated primary amines)- hydroxylammonium, NH3OH+ (protonated hydroxylamine)
- methylammonium, CH3NH+
3 (protonated methylamine) - ethylammonium, C2H5NH+
3 (protonated ethylamine) - hydrazinium, or diazanium, NH2NH+
3 (protonated hydrazine, a.k.a. diazane) - anilinium (a.k.a. phenylammonium), C
6H
5NH+
3 (protonated aniline, a.k.a. phenylamine, aminobenzene)
- 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)
- dimethylammonium (sometimes dimethylaminium), (CH3)2NH+
- tertiary ammonium cations, R3NH+ (protonated tertiary amines)
- trimethylammonium (CH3)3NH+ (protonated trimethylamine)
- triethylammonium (C2H5)3NH+ (protonated triethylamine)
- quaternary ammonium cations, R4N+ or NR+
4- tetramethylammonium, (CH3)4N+
- tetraethylammonium, (C2H5)4N+
- tetrapropylammonium, (C3H7)4N+
- tetrabutylammonium, (C4H9)4N+ or abbreviated Bu4N+
- trimethyl ammonium compounds, (CH3)3RN+
- didecyldimethylammonium, (C10H21)2(CH3)2N+
- pentamethylhydrazinium, N(CH3)2N(CH3)+
3 - tetrafluoroammonium, NF+
4
- quaternary phosphonium cations, R4P+ or PR+
4- tetraphenylphosphonium, (C
6H
5)
4P+
- tetraphenylphosphonium, (C
- primary oxonium cations, [ROH
2]+
(protonated R-O-H oxygen compounds) - secondary oxonium cations, [R
2OH]+
(protonated R-O-R oxygen compounds)- dialkyloxonium cations (protonated ethers)
- dimethyloxonium, [(CH
3)
2OH]+
(protonated dimethyl ether)
- dimethyloxonium, [(CH
- dialkyloxonium cations (protonated ethers)
- tertiary oxonium cations, [R
3O]+
- trimethyloxonium, [(CH
3)
3O]+ - triethyloxonium, [(C
2H
5)
3O]+
- trimethyloxonium, [(CH
- secondary sulfonium cations, R2SH+ (protonated sulfides)
- dimethylsulfonium, (CH3)2SH+ (protonated dimethyl sulfide)
- tertiary sulfonium cations, R3S+
- trimethylsulfonium, (CH3)3S+
- secondary fluoronium cations, R2F+
- dichlorofluoronium, Cl2F+
- trifluoroxenonium, XeF+
3 (XeF2 is neutral.)
Onium cations with polyvalent substitutions
- secondary ammonium cations having one double-bonded substitution, R=NH+
2- diazenium, HN=NH+
2 (protonated diazene) - guanidinium, (H
2N)
2C=NH+
2 (protonated guanidine) (has a resonance structure)
- diazenium, HN=NH+
- tertiary ammonium cations having one triple-bonded substitution, R≡NH+
- tertiary ammonium cations having two partially double-bonded substitutions, RNH+R
- pyridinium, C5H5NH+ (protonated pyridine)
- quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR+
2 - quaternary ammonium cations having two double-bonded substitutions, R=N=R+
- nitronium, [NO
2]+ - bis(triphenylphosphine)iminium, ((C6H5)3P)2N+
- nitronium, [NO
- quaternary ammonium cations having one triple-bonded substitution and one single-bonded substitution, R≡NR+
- diazonium, N≡NR+ (substituted protonated nitrogen)
- nitrilium, RC≡NR+ (substituted protonated nitrile)
- tertiary oxonium cations having one triple-bonded substitution, R≡O+
- acylium ions, R−C≡O+ ↔ R−C+=O
- nitrosonium, N≡O+
- tertiary oxonium cations having two partially double-bonded substitutions, RO+R
- pyrylium, C5H5O+
- tertiary sulfonium cations having one triple-bonded substitution, R≡S+
- thionitrosyl, N≡S+
Double onium dications
- hydrazinediium or hydrazinium(2+) dication, +H3NNH+
3 (doubly protonated hydrazine) - diazenediium cation, +H2N=NH+
2 (doubly protonated diazene) - diazynediium cation, +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)
Substituted eniums
- diphenylcarbenium, (C
6H
5)
2CH+ (di-substituted methenium) - triphenylcarbenium, (C
6H
5)
3C+
(tri-substituted methenium)
Ynium cations
- carbynium ions (protonated carbynes) have a divalent carbon atom with a +1 charge.
- alkynium cations, C
nH+
2n-1 (n ≥ 2) (protonated alkynes)- methynium cation, H
2C+
(protonated methylidyne radical) - ethynium, C
2H+
3 (protonated ethyne)
- methynium cation, H
- alkynium cations, C
See also
- Carbonium ion
- Lyonium ion, a protonated solvent molecule
- Lyate ion, a deprotonated solvent molecule
References
- Onium compounds definition at IUPAC Gold Book
- 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
- Carbonium ion definition at IUPAC Gold Book
- RC-82. Cations, Queen Mary University of London)
External links
- Ions and Radicals, Queen Mary University of London
- Onium compounds at the US National Library of Medicine Medical Subject Headings (MeSH)
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