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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?

Tan Yong Boon
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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.

Jan
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  • 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 – Tan Yong Boon Nov 14 '17 at 05:53
  • I'm sure Jan already knows that, as it is a key reason for the stability of carbocations β to silicon. – orthocresol Nov 14 '17 at 09:17
  • @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 ;) – Jan Nov 14 '17 at 09:25
  • Have you taken a look at the comment at my other post? – Tan Yong Boon Nov 14 '17 at 09:35
  • @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. – Tan Yong Boon Nov 25 '17 at 05:10
  • @TanYongBoon It should be $\ce{C-Pb}$ because the HOMO is most carbon-centred among the four. – Jan Nov 25 '17 at 12:39
  • Hi again! Another question: Do you think C-Br and C-I bonds would be good hyperconjugative donors? Would they be better than C-H and C-C bonds? I was thinking that despite they may be more electronegative, they form weak bonds with carbon and they are also not as electronegative as F and Cl. – Tan Yong Boon Jan 07 '18 at 09:03
  • They are not good hyperconjugators because they are polarised towards the halogen and thus away from the carbon. However, their sigma* orbitals are carbon-centred, leading to e.g. the anomeric effect. – Jan Jan 07 '18 at 15:48