What are the sigma and pi bonds in cumulene?

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What are the sigma and pi bonds in cumulene?

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Answer 1 · 2016-03-31T20:57:07

Cumulenes are a class of compounds with $C = C$ $"C"="C"$ chains directly connected (like $\cdot \cdot \cdot = C = C = C = C = \cdot \cdot \cdot$ $cdotcdotcdot="C"="C"="C"="C"=cdotcdotcdot$ ).

I can't assume that you are necessarily talking about a particular cumulene, but given the examples in the wikipedia page, I'm going to assume that you are talking about a cumulene that only contains the $C = C$ $"C"="C"$ chain, but nothing crazy like ${(CO)}_{5} M = C = C = C = C = C = C = C - {({NMe}_{2})}_{2}$ $("CO")_5"M"="C"="C"="C"="C"="C"="C"="C"-("NMe"_2)_2$ like I would find in my textbook chapter on metallacumulenes. :)

In that case, there would be one $σ$ $sigma$ bond and one $π$ $pi$ bond for each $C = C$ $"C"="C"$ connection, as a pure double bond would contain one $σ$ $sigma$ and one $π$ $pi$ bond. There would also be one $σ$ $sigma$ bond for each terminal $C - H$ $"C"-"H"$ .

If we were to take butatriene as an example (coincidentally also called cumulene... see what I mean?), then, seeing as it is:

$H_{2} C = C = C = {CH}_{2}$ $\mathbf("H"_2"C"="C"="C"="CH"_2)$

...there are three $C - C$ $"C"-"C"$ $π$ $pi$ bonds, three $C - C$ $"C"-"C"$ $σ$ $sigma$ bonds, and four $C - H$ $"C"-"H"$ $σ$ $sigma$ bonds overall.

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What are the sigma and pi bonds in cumulene?

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