What is a non-classical carbocation?

* What is a non-classical carbocation?
* How is it different from a classical carbocation?


I am confused as I have come across this term many times on Chem.SE but there seems to be nothing for my level of understanding on the Internet!




Here is a picture of a "classical" carbocation, there is an electron deficient carbon bearing a positive charge.


![enter image description here](https://i.stack.imgur.com/IfdFC.png)


There are many examples of "non-classical" carbocations, but the [2-norbornyl carbocation](http://en.wikipedia.org/wiki/2-Norbornyl_cation) is among the best known.


[![Norbornyl](https://i.stack.imgur.com/f2mqy.png)](https://i.stack.imgur.com/f2mqy.png)


Labeling experiments have shown that the positive charge resides on more than one carbon in the 2-norbornyl ion. Early on, the data was explained by equilibrating classical ions, but soon another possibility emerged - one involving a single non-classical ion.


The problem comes down to: are the equilibrating classical ions ground state structures with the non-classical ion serving as the transition state, or is the non-classical ion the ground state? This debate went on for a very long period of time, but now most agree that the non-classical structure is the ground state in the 2-norbornyl system. In fact, a recent, and difficult to obtain, [crystal structure for the 2-norbornyl cation](http://www.sciencemag.org/content/341/6141/62) has been published proving that the ion exists with the non-classical geometry (thanks to Klaus for finding this reference, see his comment below). 


![enter image description here](https://i.stack.imgur.com/0MoLl.png)


**A key difference between classical and non-classical structures is the bonding**. As illustrated above, a classical ion has a carbon with a sextet of electrons and 3 other bonds. The non-classical ion, on the other hand, involves 3 carbons with 2 electrons spread over them. This is called a [3-center 2-electron bond](http://en.wikipedia.org/wiki/Three-center_two-electron_bond) ([hypercoordinate bonding](http://en.wikipedia.org/wiki/Hypervalent_molecule)) and is **a clear marker for a non-classical ion**. Notice if you count all of the bonds to any of these 3 carbon atoms (solid and dashed lines) you count 5! Sounds strange, but such "hypercoordinate bonding" is a permitted consequence of the 3-center 2-electron bond.


[![enter image description here](https://i.stack.imgur.com/LVZBR.png)](https://i.stack.imgur.com/LVZBR.png)


Here is a [link](https://chemistry.stackexchange.com/questions/29000/which-carbocation-is-more-stable-the-ethyl-or-1-propyl-carbocation/31729#31729) to a simpler non-classical carbocation discussed here yesterday.


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