In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom. They are a type of carbocation. In older literature, the name "carbonium ion" was used for what is today called carbenium. Carbonium ions charge is delocalized in three-center, two-electron bonds. The more stable members are often bi- or polycyclic.

2-Norbornyl cation

The 2-Norbornyl cation is one of the best characterized carbonium ion. It is the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography, it has a symmetric structure with an RCH2+ group bonded to an alkene group, stabilized by a bicyclic structure.

Cyclopropylmethyl cation

A non-classical structure for is supported by substantial experimental evidence from solvolysis experiments and NMR studies. One or both of two structures, the cyclopropylcarbinyl cation and the bicyclobutonium cation, were invoked to account for the observed reactivity. he NMR spectrum consists of two 13C NMR signals, even at temperatures as low as −132 °C. Computations suggest that the energetic landscape of the system is very flat. The bicyclobutonium structure is computed to be 0.4 kcal/mol more stable than the cyclopropylcarbinyl structure. In the solution phase (SbF5·SO2ClF·SO2F2, with as the counterion), the bicyclobutonium structure predominates over the cyclopropylcarbinyl structure in a 84:16 ratio at −61 °C. Three other possible structures, two classical structures (the homoallyl cation and cyclobutyl cation) and a more highly delocalized non-classical structure (the tricyclobutonium ion), are less stable. The low temperature NMR spectrum of a dimethyl derivative shows two methyl signals, indicating that the molecular conformation of this cation is not perpendicular (as in A), which possesses a mirror plane, but is bisected (as in B) with the empty p-orbital parallel to the cyclopropyl ring system: In terms of bent bond theory, this preference is explained by assuming favorable orbital overlap between the filled cyclopropane bent bonds and the empty p-orbital.

Methanium and ethanium

The simplest carbonium ions are also the least accessible. In methanium (CH5+) carbon is covalently bonded to five hydrogen atoms. The ethanium ion C2H7+ has been characterized by infrared spectroscopy. The isomers of octonium (protonated octane, C8H19+) have been studied.

Pyramidal carbocations

Applications

Carbonium ions are intermediates in the isomerization of alkanes catalyzed by very strong solid acids. Such carbonium ions are invoked in cracking (Haag-Dessau mechanism).