Onium ion
Class of positively-charged molecules
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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]
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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:
- 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]
Periodic table
Onium ions by group
Group 13 (boron group) onium cations
Group 14 (carbon group) onium cations
- Carbonium ions (protonated hydrocarbons) have a pentacoordinated carbon atom with a +1 charge. The specific cation CH+5 is called methanium.[4]. Typically named for the parent hydrocarbon, e.g. C2H+7 is ethanium.[5]
- silanium (sometimes silonium), SiH+5 (protonated silane) (should not be called siliconium[6])
- germonium, GeH+5 (protonated germane) Unstable derivative known of R3Ge+.[7][8]
- stannonium, SnH+3 (protonated stannylene, SnH2) (not protonated stannane SnH4) stable at cryogenic conditions.[9]
Group 15 (pnictogen) onium cations
- ammonium (IUPAC name azanium), NH+4 (protonated ammonia (IUPAC name azane)) exists in aqueous solution and as salts.
- primary, secondary, and tertiary organic derivatives [NHnR4−n]+, derived from protonation of amines; e.g. methylammonium halides, diethylammonium chloride, trimethylammonium
- quaternary ammonium cations NR+4; e.g. tetrabutylammonium
- phosphonium, PH+4 (protonated phosphine)
- primary, secondary, and tertiary organic derivatives [PHnR4−n]+, derived from protonation of phosphines
- quaternary phosphonium cations PR+4; e.g. tetraphenylphosphonium
- arsonium, AsH+4 (protonated arsine) known as derivatives or as an unstable salt at cryogenic temperatures.[10]
- stibonium, SbH+4 (protonated stibine) known as derivatives but no salts are known.[10]
- bismuthonium, BiH+4 (protonated bismuthine) only known as derivatives.[11]
Group 16 (chalcogen) onium cations
- oxonium, H3O+ (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.)
- Organic derivatives can be primary (ROH+2, protonated alcohols), secondary (R2OH+, protonated ethers), or ternary (R3O+, as trimethyloxonium).
- peroxonium, H3O+2 (protonated hydrogen peroxide)
- sulfonium, H3S+ (protonated hydrogen sulfide)
- Organic derivatives can be primary (H2SR+, protonated thiols), secondary (HSR+2, protonated thioethers), or ternary (SR+3, e.g. trimethylsulfonium)
- selenonium, H3Se+ (protonated hydrogen selenide)
- Tertiary organic derivatives R3Se+ are known, with trimethylselenonium iodide being the first.[12]
- telluronium, H3Te+ (protonated hydrogen telluride)
Hydrogen onium cation
- hydrogenonium, better known as trihydrogen cation, H+3 (protonated molecular or diatomic hydrogen), found in ionized hydrogen and interstellar space
Group 17 (halogen) onium cations, halonium ions, H2X+ (protonated hydrogen halides)
- fluoronium, H2F+ (protonated hydrogen fluoride)
- chloronium, H2Cl+ (protonated hydrogen chloride)
- bromonium, H2Br+ (protonated hydrogen bromide)
- iodonium, H2I+ (protonated hydrogen iodide)
Pseudohalogen onium cations
- aminodiazonium, [H2N=N=N]+ ⇌ [H2N−N≡N]+ (protonated hydrogen azide)
- methylidyneammonium and hydrocyanonium, H2CN+, isomers HC≡NH+ ⇌ N≡CH+2 (protonated hydrogen cyanide)
Group 18 (noble gas) onium cations
- hydrohelium or helonium, better known as helium hydride ion, HeH+ (protonated helium) known in gas phase.[13]
- neonium, NeH+ (protonated neon) known in gas phase.[14]
- argonium, ArH+ (protonated argon) known in gas phase.[15]
- kryptonium, KrH+ (protonated krypton) known in gas phase.[16]
- xenonium, XeH+ (protonated xenon) known in gas phase.[17]
Onium cations with monovalent substitutions
- tertiary selenonium cations, R3Se+
- triphenylselenonium, (C6H5)3Se+[18]
- tertiary telluronium cations, R3Te+
- triphenyltelluronium, (C6H5)3Te+[19]
- primary fluoronium cations, RFH+ (protonated fluorides RF)
- secondary fluoronium cations, R2F+
- dichlorofluoronium, Cl2F+
- secondary iodonium cations, R2I+
- diphenyliodonium, (C6H5)2I+
Onium cations with polyvalent substitutions
- secondary ammonium cations having one double-bonded substitution, R=NH+2
- diazenium, HN=NH+2 (protonated diazene)
- guanidinium, C(NH2)+3 (protonated guanidine) (has a resonance structure and a planar molecular geometry)
- 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)
- 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, [NO2]+
- bis(triphenylphosphine)iminium, ((C6H5)3P=)2N+
- 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+
- acylium ions, R−C≡O+ ↔ R−C+=O
- nitrosonium, N≡O+
- tertiary sulfonium cations having one triple-bonded substitution, R≡S+
- thionitrosyl, N≡S+
- dihydroxyoxoammonium, [H2NO3]+ (protonated nitric acid)
- trihydroxyoxosulfonium, [H3SO4]+ (protonated sulfuric acid)
- cyclic tertiary onium cations
- pyrylium, C5H5O+
- thiopyrylium, C5H5S+[20]
- selenopyrylium, C5H5Se+[20]
- telluropyrylium, C5H5Te+[20]
Double onium dications
- hydrazinediium or hydrazinium(2+) dication, H3N+−+NH3 (doubly protonated hydrazine, in other words, doubly protonated diazane)
- diazenediium cation, H2N+=+NH2 (doubly protonated diazene)
- diazynediium cation, HN+≡+NH (doubly protonated dinitrogen, in other words, doubly protonated diazyne)
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, R2B+ (protonated borylenes a.k.a. boranylidenes)
- carbenium cations, R3C+ (protonated carbenes) have a tricoordinated carbon atom with a +1 charge.
- silylium cations, R3Si+ (protonated silylenes)
- nitrenium cations, R2N+ (protonated nitrenes)
- phosphinidenium cations, R2P+ (protonated phosphinidene)
- mercurinium cations, R3Hg+ (protonated organomercury compounds; formed as intermediates in oxymercuration reactions)
Substituted eniums
- diphenylcarbenium, (C6H5)2CH+ (di-substituted methenium)
- triphenylcarbenium, (C6H5)3C+ (tri-substituted methenium)
Ynium cations
- carbynium ions (protonated carbynes) have a carbon atom with a +1 charge.
- alkynium cations, C
nH+
2n-1 (n ≥ 2) (protonated alkynes)- methynium cation, H2C+ (protonated methylidyne radical)
- ethynium, C2H+3 (protonated ethyne)
- alkynium cations, C
See also
- Carbonium ion
- Lyonium ion, a protonated solvent molecule
- Lyate ion, a deprotonated solvent molecule