Octonium
Carbonium ion
Any cation that has a pentavalent carbon atom
In chemistry, a carbonium ion is any cation that has a pentacoordinated carbon atom.[1][2] The name carbonium may also be used for the simplest member of the class, properly called methanium (CH+5), where the carbon atom is covalently bonded to five hydrogen atoms.[3][4][5][6]
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.[7] The isomers of octonium (protonated octane, C8H+19) have been studied.[8] 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[1] and are now widely accepted.
A stable carbonium ion is the complex pentakis(triphenylphosphinegold(I))methanium (Ph3PAu)5C+, produced by Schmidbauer and others.[9]