Class of positively-charged molecules
Not to be confused with
onium states such as positronium.
Inchemistry , anonium ion is acation formally obtained by theprotonation of mononuclear parenthydride of apnictogen (group 15 of theperiodic table ),chalcogen (group 16), orhalogen (group 17). The oldest-known onium ion, and the namesake for the class, isammonium ,NH+ 4 , the protonated derivative ofammonia ,NH3 .[ 1] [ 2]
The nameonium 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 astetraphenylphosphonium ,(C6 H5 )4 P+ . The substituent groups may be divalent or trivalent, yielding ions such asiminium andnitrilium .[ 1] [ 2]
A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called adouble onium ion , and has a charge of +2. Atriple onium ion has a charge of +3, and so on.
Compounds of an onium cation and some otheranion are known asonium compounds oronium salts .
Onium ions and onium compounds are inverselyanalogous to-ate ate complexes :
Lewis bases form onium ions when the central atom gains one more bond and becomes a positivecation .Lewis acids form-ate anion .[ 3] [ edit ] carbonium ions (protonatedhydrocarbons ) have a pentacoordinated carbon atom with a +1 charge.alkanium cations,Cn + 2n +3 (protonatedalkanes )methanium ,CH+ 5 (protonatedmethane ) (Sometimes calledcarbonium , because it is the simplest member of that class, but that use is deprecated because of multiple definitions.[ 4] ethanium ,C2 H+ 7 (protonatedethane )propanium ,C3 H+ 9 (propane protonated on an unspecified carbon)butanium ,C4 H+ 11 (butane protonated on an unspecified carbon)octonium or octanium,C8 H+ 19 (protonatedoctane ) silanium (sometimessilonium ),SiH+ 5 (protonatedsilane ) (should not be called siliconium[ 5] germonium ,GeH+ 5 (protonatedgermane )stannonium ,SnH+ 3 (protonatedSnH2 ) (not protonatedstannane SnH4 )plumbonium ,PbH+ 3 (protonatedPbH2 ) (not protonatedplumbane PbH4 )flerovonium ,FlH+ 3 (protonatedFlH2 ) (not protonatedflerovane FlH4 )Hydrogen onium cation [ edit ] Onium cations with monovalent substitutions [ edit ] primary ammonium cations,RH3 N+ orR−NH+ 3 (protonated primaryamines ) secondary ammonium cations,R2 NH+ 2 (protonated secondaryamines ) tertiary ammonium cations,R3 NH+ (protonated tertiaryamines ) quaternary ammonium cations ,R4 N+ orNR+ 4 quaternary phosphonium cations,R4 P+ orPR+ 4 quaternary arsonium cations,R4 As+ orAsR+ 4 quaternary stibonium cations,R4 Sb+ orSbR+ 4 primary oxonium cations,ROH+ 2 (protonatedalcohols R−O−H )alkyloxonium cationsROH+ 2 (protonatedalcohols )methyloxonium,CH3 OH+ 2 (protonatedmethanol ) ethyloxonium,CH3 CH2 OH+ 2 (protonatedethanol ) dioxidanonium (hydroxylhydronium),HO−OH+ 2 (protonatedhydrogen peroxide ) secondary oxonium cations,R2 OH+ (protonatedethers R−O−R )dialkyloxonium cations (protonatedethers ) tertiary oxonium cations,R3 O+ primary sulfonium cations,RSH+ 2 (protonatedthiols R−S−H ) secondary sulfonium cations,R2 SH+ (protonatedthioethers R−S−R ) tertiary sulfonium cations,R3 S+ tertiary selenonium cations,R3 Se+ tertiary telluronium cations,R3 Te+ primary fluoronium cations,RFH+ (protonatedfluorides RF) secondary fluoronium cations,R2 F+ secondaryiodonium cations,R2 I+ Onium cations with polyvalent substitutions [ edit ] secondary ammonium cations having one double-bonded substitution,R=NH+ 2 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 bearomatic )quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions,R=NR+ 2 iminium ,R2 C=NR+ 2 (substituted protonatedimine )diazenium,RN=NR+ 2 (substituted protonated diazene) thiazolium,[C3 NSR4 ]+ (substituted protonatedthiazole ) quaternary ammonium cations having two double-bonded substitutions,R=N+ =R 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+ cyclic tertiary oxonium cations where oxygen is a member of a ring,RO+ R (the ring may be aromatic) tertiary sulfonium cations having one triple-bonded substitution,R≡S+ dihydroxyoxoammonium ,[H2 NO3 ]+ (protonatednitric acid )trihydroxyoxosulfonium ,[H3 SO4 ]+ (protonatedsulfuric acid )Double onium dications [ edit ] The extra bond is added to a less-common parent hydride, acarbene analog , typically named-ene or-ylene , which is neutral with 2 fewer bonds than the more-common hydride, typically named-ane or-ine .
diphenylcarbenium,(C6 H5 )2 CH+ (di-substituted methenium) triphenylcarbenium ,(C6 H5 )3 C+ (tri-substituted methenium)
