Is a resonance structure valid if an atom has less than an octet?

[stester77s]

Hello, we know a resonance structure is not valid if say a carbon has 5 bonds. That is more than the octet, and not valid.

But what if a Carbon atom has 3 bonds, no lone pairs, and then a + charge. I saw one example in a textbook where it showed a "valid" resonance structure which had a carbon with 3 bonds and a + charge . But I thought it had to meet the octet, and coming short or having too many bonds was not valid. Why is less than an octet okay, but more than an octet not okay?

Thanks!

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[DrknoSDN]

Resonance structures are just a way of representing all the possible conformations that can contribute to the molecular behavior.
So a very minor resonance structure could include a carbocation with the electrons delocalized somewhere else in the molecule (temporarially).

Remember that all resonance structures do not contribute equally to the behavior of the molecule. Good diagrams will show %Contribution.

 

[Teleologist]

Not every atom obeys the octet rule. Take the group 3A elements as an example. Also, we can have resonance contributors with atoms "breaking" the octet rule.

 

[NextStepTutor_1]

Going off what what DrknoSDN said, not only is a carbon with a positive charge a very minor resonance structure, it's also theoretically possible. The carbon atom can be bonded to 3 different atoms and still be missing electrons to fulfill it's octet. However, that is not the case for a carbon with 5 bonds--it's theoretically impossible. A carbon with 5 bonds would mean that that carbon had 10 electrons in it's outer shell, which is impossible for carbon.

 

[Teleologist]

Of course it's possible for non-ideal structures to be resonance contributors. Consider the below diagram for the formamide molecule. Hell, we even have some ionic character.

 

[Teleologist]

However, that is not the case for a carbon with 5 bonds--it's theoretically impossible. A carbon with 5 bonds would mean that that carbon had 10 electrons in it's outer shell, which is impossible for carbon.

http://pubs.acs.org/doi/abs/10.1021/j100347a035

 

[DrknoSDN]

http://pubs.acs.org/doi/abs/10.1021/j100347a035

I think that article is talking about Cyclic 5 carbon rings, instead of Carbons with 5 covalent bonds that NextStep was talking about.

Only image that would show a carbon with 5 bonds would be a theoretical transition state of something like a SN2 substitution. Just google images for "SN2 transition state".