Why is a bridgehead atom bearing a negative or a positive charge unstable?

Why is a bridgehead atom bearing a negative or a positive charge unstable? Is it something with the structure of bridge bonds? I am unable to understand why.


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




Since the [answer by Aditya Dev](https://chemistry.stackexchange.com/a/36955/5026) has already dealt with the carbocation part, I'll write about the carbanion one. The OP states that:



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> Why is a bridgehead atom bearing a negative or a positive charge unstable?
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This is partially incorrect. Carbanions in bridgehead position are infact stable. From March Advanced Organic Chemistry, 6th ed, Chapter 5, Carbanions:



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> Carbons at bridgeheads, although extremely reluctant to undergo reactions in which they must be converted to carbocations, undergo with ease reactions in which they must be carbanions and **stable bridgehead carbanions are known.**
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(my emphasis)


It also says that:



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> For other evidence that carbanions are pyramidal, see Streitwieser, Jr., A.; Young, W.R. J. Am. Chem. Soc. 1969, 91, 529; Peoples, P.R.; Grutzner, J.B. J. Am. Chem. Soc. 1980, 102, 4709.
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This makes the picture completely clear. Bridgehead carbanions are not planar, but in fact, pyramidal. So, they are **definitely** not sp2 and hence, the Bredt rule isn't violated by their being stable.


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The hybridisation of a bridgehead carbon when its not charged is sp3. Now, if you remove a hydrogen atom, its hybridisation becomes sp2. 


Normally, in sp2 hybridisation, the 3 hybridised orbitals lie in a single plane for minimised repulsions. But the bridgehead carbon with positive charge would not have hybridised orbitals in a single plane. This increases repulsion and makes the compound unstable. 


Here is a link - [Bredt's Rule](http://www.masterorganicchemistry.com/2014/09/02/bredts-rule-and-summary-of-cycloalkanes/) - that explains the same thing, but instead of a positively charged bridgehead, they discuss a double bond on the bridgehead atom. 


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