In chemistry, a carbonium ion is any cation that has a pentacoordinated carbon atom. The name carbonium may also be used for the simplest member of the class, properly called methanium (.mw-parser-output .template-chem2-su{display:inline-block;font-size:80%;line-height:1;vertical-align:-0.35em}.mw-parser-output .template-chem2-su>span{display:block;text-align:left}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em}CH+5), where the carbon atom is covalently bonded to five hydrogen atoms.
The next simplest carbonium ions after methanium have two carbon atoms. Ethynium, or protonated acetylene C2H+3, and ethenium C2H+5 are usually classified in other families. The ethanium ion C2H+7 has been studied as an extremely rarefied gas by infrared spectroscopy. The isomers of octonium (protonated octane, C8H+19) have been studied. The carbonium ion has a planar geometry.
In older literature, the name "carbonium ion" was used for what is today called carbenium. The current definitions were proposed by the chemist George Andrew Olah in 1972 and are now widely accepted.
A stable carbonium ion is the complex pentakis(triphenylphosphinegold(I))methanium (Ph3PAu)5C+, produced by Schmidbauer and others.
Carbonium ions can be obtained by treating alkanes with very strong acids. Industrially, they are formed in the refining of petroleum during primary thermal cracking (Haag-Dessau mechanism).