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.
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, . The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium.
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.
Simple onium cations (hydrides with no substitutions)
Group 15 (pnictogen) onium cations
Group 16 (chalcogen) onium cations
Group 14 (carbon group) onium cations
- carbonium ions (protonated hydrocarbons) have a pentavalent carbon atom with a +1 charge.
- alkanium cations, (protonated alkanes)
- methanium, (protonated methane) (Sometimes called carbonium, because it is the simplest member of that class, but that use is deprecated because of multiple definitions. Sometimes called methonium, but methonium also has multiple definitions. Abundant in outer space.)
- ethanium, (protonated ethane)
- propanium, (propane protonated on an unspecified carbon)
- butanium, (butane protonated on an unspecified carbon)
- octonium or octanium, (protonated octane)
- silanium (sometimes silonium), (protonated silane. Should not be called siliconium.
- further silanium cations, (protonated silanes)
- germonium, (protonated germane)
- stannonium, (protonated ; not protonated stannane )
- plumbonium, (protonated )
Group 13 (boron group) onium cations
Group 18 (noble gas) onium cations
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)
- secondary ammonium cations, R2NH+
2 (protonated secondary amines)
- tertiary ammonium cations, R3NH+ (protonated tertiary amines)
- quaternary ammonium cations, R4N+ or NR+
- quaternary phosphonium cations, R4P+ or PR+
- primary oxonium cations, (protonated R-O-H oxygen compounds)
- alkyloxonium cations (protonated alcohols)
- secondary oxonium cations, (protonated R-O-R oxygen compounds)
- dialkyloxonium cations (protonated ethers)
- tertiary oxonium cations,
- secondary sulfonium cations, R2SH+ (protonated sulfides)
- tertiary sulfonium cations, R3S+
- secondary fluoronium cations, R2F+
- trifluoroxenonium, XeF+
3 (XeF2 is neutral.)
Onium cations with polyvalent substitutions
- secondary ammonium cations having one double-bonded substitution, R=NH2+
- tertiary ammonium cations having one triple-bonded substitution, R?NH+
- tertiary ammonium cations having two partially double-bonded substitutions, RNH+R
- quaternary ammonium cations having one double-bonded substitution and two single-bonded substitutions, R=NR2+
- iminium, R2C=NR2+ (substituted protonated imine)
- diazenium, RN=NR2+ (substituted protonated diazene)
- thiazolium, (substituted protonated thiazole)
- 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)
- nitrilium, RC?NR+ (substituted protonated nitrile)
- tertiary oxonium cations having one triple-bonded substitution, R?O+
- tertiary oxonium cations having two partially double-bonded substitutions, RO+R
- tertiary sulfonium cations having one triple-bonded substitution, R?S+
Double onium dications
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.
- diphenylcarbenium, (di-substituted methenium)
- triphenylcarbenium, (tri-substituted methenium)