Hybridization of sulfur in sulfur dioxide One of the canonical structures for sulfur dioxide - $\ce{SO2}$ - has sulfur (with a lone electron pair) double bonded to each oxygen atom to form a total of 4 bonds for sulfur - which can be achieved via valence expansion into empty d-orbitals.

What then is the hybridization of the valence-expanded sulfur?
It is described as sp². But how can that be? This seems unlikely
because d-orbitals are involved since the sulfur underwent valence
expansion.

On might imagine a pair of electrons from the 3s/3p oribital(s) being promoted to an empty d-orbital and then having the 3s and 3p orbitals hybridize in to sp².
If this is true it would mean that the lone electron pair of the
valence-expanded sulfur consists of 2 electrons occupying
and unhybridized d-orbital. But is this correct?
The structure of sulfur dioxide ($\ce{SO2}$) is quite complicated.
The image from socratic.org* explains it quite well, please find the $\ce{SO2}$ hybridization diagram via the Internet Archive.
As seen, all the atoms have $\ce{sp^2}$ hybridization.
I'll only focus on the central sulfur atom.

Two $\ce{sp^2}$ orbitals form $\ce{\sigma}$-bonds with the two oxygens.
The other $\ce{sp^2}$ orbital is where the lone pair lives in.

Now, we have dealt with 4 electrons, and only have 2 electrons to deal with.
The 2 remaining electrons actually live in the unhybridized $\ce{p}$ orbital.
At this point, the two oxygen atoms will also have 2 electrons left to pair with the sulfur atom.

The two oxygen atoms get to keep their own electron.
Sulfur shares two electrons among itself and the other two oxygen atoms.

Thus, no electron lives in the $\ce{d}$ orbital.


$\color{Red}{\mbox{red}}$ represents number of electrons (I am too lazy to draw the fish-hooks).
$\color{Green}{\mbox{green}}$ represents $\ce{sp^2}$ orbital.
$\color{blue}{\mbox{Blue}}$ represents $\ce{p}$ orbital.

Note that the two $\ce{sp^2}$ orbitals between sulfur and the two oxygen atoms are in $\ce{\sigma}$-bond.

* The website is no longer available; there is no version in the Internet Archive.