Editing Carbocation (section)

===Carbenium ions===
{{Main|Carbenium ion}}
At least in a formal sense, carbenium ions (CR<sub>3</sub><sup>+</sup>) are derived from the protonation (addition of {{chem2|H+}}) or alkylation (addition of {{chem2|R+}}) of a [[carbene]] or [[alkene]].  They admit a [[Resonance (chemistry)|resonance]] depiction in which one carbon atom bears a formal positive charge and is surrounded by six [[valence electron]]s instead of the [[octet rule|usual octet]]. Therefore, carbenium ions (and carbocations in general) are often reactive, seeking to fill the valence octet and regain a neutral [[electric charge|charge]]. 

In accord with [[VSEPR theory|VSEPR]] and [[Bent's rule]], unless geometrically constrained to be pyramidal (e.g., 1-adamantyl cation), 3-coordinate carbon in carbenium ions are usually trigonal planar and [[orbital hybridization|{{math|''sp''<sup>2</sup>}} hybridized]]; the [[LUMO|lowest unoccupied molecular orbital]] is an empty pure ''p'' orbital pointing out-of-plane.  A prototypical example is the ''t-''butyl cation, {{chem2|CMe3+}}.  Although classical carbenium ions have a structure that corresponds to a non-bridging Lewis structure, it is important to note that donation of electron density from neighboring C–H or C–C bonds into the "empty" p orbital, known as [[hyperconjugation]], is still an important stabilizing factor, and these bonds have a tendency to "lean" towards the carbocationic center to improve orbital overlap.  

There is, in fact, an entire spectrum of bonding scenarios between a slight lean due to hyperconjugation to a fully symmetric bridging structure featuring 3c2e bonding.  Consequently, there is no firm dividing line between "classical" and the so-called "non-classical" structures. <!--For the same reasons, carbocations that are 2-coordinate (vinyl cations) are generally linear in geometry, with CH/CC bonds formed from C(sp) orbitals.-->

==== Non-classical carbenium ions ====
Non-classical carbenium ions feature also σ delocalization (3c2e bonds) in their bonding but have the general formula CR<sub>3</sub><sup>+</sup> (R = alkyl or H).  Thus, in principle, one can propose non-bridged, classical structures for these cations, as well as a bridged non-classical structure.  Because of the subtle differences in the expected behavior of a non-classical carbenium ions compared to the alternative hypothesis of two rapidly equilibrating classical structures, a lively and often acrimonious debate took place over several decades regarding the merits of each model. For a detailed history of this dispute, see the article on the [[2-Norbornyl cation|2-norbornyl cation]]. Currently, there is overwhelming evidence that, at least in some cases (notably the extremely well-studied 2-norbornyl cation), the equilibrium structure of a carbenium ion is non-classical, although even minor changes in structure could result in a classical structure being favored.