Carbonium ion

In chemistry, carbonium ion is any cation that has a pentavalent carbon atom, The name carbonium may also be used for the simplest member of the class, properly called methanium (CH+
5), where the five valences are filled with hydrogen atoms.

The next simplest carbonium ions after methanium have two carbon atoms. Ethynium or protonated acetylene C
2H+
3 and ethenium C
2H+
5 are usually classified in other families. The ethanium ion C
2H+
7 has been studied as an extremely rarefied gas by infrared spectroscopy.

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 penta(triphenylphosphine gold(I))methanium (Ph
3PAu)
5C+
, produced by Schmidbauer and others.

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