Carbon/silicon pentacovalent intermediate during substitution

During SN2 reaction with a silicon compound, a pentacovalent intermediate is formed, while there is no such intermediate formed with carbon compounds (only a pentacovalent transition state). Why is it so? I need an MO explanation (no d orbitals involved).




Bonds involving carbon are typically shorter than bonds involving silicon. Therefore a pentacoordinated carbon species would be **sterically more crowded** around the central carbon atom than its pentacoordinated silicon analogue would be. This crowding would raise the energy of pentacoordinate carbon compared to pentacoordinate silicon; which is consistent with the observation that the SN2 reaction with carbon produces a higher energy pentacoordinate transition state, while in the case of silicon a more stable pentacoordinate intermediate can be observed.


\begin{array}{lcc}
\hline
\ce{Bond} & d(\ce{X=C})/\pu{pm} & d(\ce{X=Si})/\pu{pm} \\
\hline
\ce{X-O} & 143 & 163 \\
\ce{X-Cl} & 177 & 202 \\
\ce{X-Br} & 194 & 215 \\
\ce{X-I} & 214 & 243 \\
\hline
\end{array}


**Recasting this into MO terms**, we would say that both the carbon and silicon pentacoordinate species involve a trigonal bipyramid structure where the two ligands (the "attacking" and "leaving" groups in the carbon case) occupy axial positions in the trigonal bipyramid arrangement.


[![Structure of the SN2 transition state](https://i.stack.imgur.com/UPqm0.png)](https://i.stack.imgur.com/UPqm0.png)


([source](https://en.wikipedia.org/wiki/SN2_reaction))


The figure above makes it easy to see that we have a [hypercoordinated](https://en.wikipedia.org/wiki/Hypervalent_molecule) species involving a 3-center 4-electron bond. Since a silicon p-orbital is larger than a carbon p-orbital, the 4 electrons are spread out over a larger volume in the silicon case, reducing electron-electron repulsion. This makes the pentacoordinate silicon case more stable than pentacoordinate carbon.


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