Hyperconjugation is often illustrated with C-C and C-H bonds donating electron density to the p orbital in the adjacent carbon atom. Since the concept revolves around sigma bonding orbitals donating electron density to p orbitals, would other bonds such as Si-Si or Si-H bonds be able to take part in hyperconjugation as well? If they are not able to do so, why not?


1 Answer 1


Your description of hyperconjugation is a very minimalistic one. More generally, hyperconjugation should be seen as the interaction of occupied bonding MOs with unoccupied ones — these unoccupied ones can be both antibonding MOs or empty (cationic) p atomic orbitals.

In principle, $\ce{Si-H}$ or $\ce{Si-Si}$ bonds would also be able to donate electron density in a hyperconjugative matter. In pracitce, due to silicon’s low electronegativity and greater size these orbitals simply do not overlap as well and their energy levels are such that less stabilisation may be obtainable.

Carbon-silicon bonds are of course great at providing hyperconjugation since silicon — much like hydrogen — has a lower electronegativity than carbon, meaning the occupied orbitals have a greater contribution on carbon.

  • $\begingroup$ According to a set of slides Ron attached in one of his previous answers, the C-Si bond is a better hyperconjugation donor than the C-C and C-H bonds because it is higher in energy, for your information $\endgroup$ Nov 14, 2017 at 5:53
  • $\begingroup$ I'm sure Jan already knows that, as it is a key reason for the stability of carbocations β to silicon. $\endgroup$
    – orthocresol
    Nov 14, 2017 at 9:17
  • $\begingroup$ @TanYongBoon Your question does not mention the $\ce{C-Si}$ bond so I was confused about your comment at first. But it all falls into place looking at it properly ;) $\endgroup$
    – Jan
    Nov 14, 2017 at 9:25
  • $\begingroup$ Have you taken a look at the comment at my other post? $\endgroup$ Nov 14, 2017 at 9:35
  • $\begingroup$ @Jan Comparing between C-C, C-Si, C-Ge, and C-Pb, which is the best hyperconjugative donor? I would think it is C-Pb because it is the weakest of the 4 bonds and thus has the highest-energy HOMO. $\endgroup$ Nov 25, 2017 at 5:10

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