Carbenium ion

A carbenium ion is a positive ion with the structure RR′R″C⁺, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. But IUPAC confuses coordination number with valence, incorrectly considering carbon in carbenium as trivalent.

In older literature the name carbonium ion was used for this class, but now it refers exclusively to another family of carbocations, the carbonium ions, where the charged carbon is incorrectly considered pentavalent by the IUPAC, but it is pentacoordinated, not pentavalent, because the coordination number and valence are two different things. The current definitions were proposed by the chemist George Andrew Olah in 1972, and are now widely accepted.

Carbenium ions are generally highly reactive due to having an incomplete octet of electrons; however, certain carbenium ions, such as the tropylium ion, are relatively stable due to the positive charge being delocalised between the carbon atoms.