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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?

Mithoron
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Mark Adams
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    The role of d orbital in sulfur bonding is a controversial issue. It seems to me that the problem is not about if d orbital helps, rather than whether the role of d orbital should be regarded as a polarization function. – Rodriguez May 14 '16 at 05:56

2 Answers2

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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.

SO2 diagram

  • $\color{Red}{\mbox{red}}$ 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.

Martin - マーチン
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Kenny Lau
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    We have been taught that $SO_2$ had one $ d\pi - p\pi$ bond and one $p\pi - p\pi$ bond. So is this wrong? – jonsno Jul 16 '17 at 13:50
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    @samjoe $d$ orbital is hardly used, even in $\ce{SO3}$. – Kenny Lau Jul 16 '17 at 16:09
  • @KennyLau Does ozone follow the same analysis? If so, does this mean the central O in ozone allocates electrons differently to the end oxygens? If this is true, are there any significant consequences? – Yushi Li Apr 19 '21 at 09:14
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    @YushiLi I'm sorry, but this was 5 years ago, and I've forgotten all chemistry since then. – Kenny Lau Apr 19 '21 at 09:30
  • @KennyLau then what is hybridization of S in SO3? If you search on google, I will get more than 10 website saying that both SO2 and SO3 are sp2 and not a single one except this saying something else – Ritil Jul 11 '22 at 05:28
  • @Ritil Really? I can see there clearly just that... – Mithoron Jul 12 '22 at 21:01
  • @Mithoron It could be that due to my search history and preferences, Google shows me only such kind of websites which shows sp² hybridisation. I can give examples – Ritil Jul 12 '22 at 21:04
  • @Ritil This answer says sp2 and others too, because it is approximately sp2! – Mithoron Jul 12 '22 at 21:09
  • @Mithoron APPROXIMATELY! – Ritil Jul 13 '22 at 15:15
  • @Ritil In SO2 it's approximate, because lone pair isn't oxygen. SO3 on the other hand is more symmetric. BTW I don't get the point of this conversation. – Mithoron Jul 13 '22 at 15:55
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The canonical structure for sulphur dioxide nowadays has charge separation, one oxygen bonded to sulphur in a single bond, the other in a double bond. But not too long ago the canonical structure was indeed what you proposed with one pp-π bond and one dp-π bond.

This was generally explained with one of sulphur’s 3d-orbitals taking part in hybridisation giving rise to a ‘sp²d’ state. Taking Kenny’s scheme, though, sulphur’s 3d-orbitals are a good step above the 3p orbitals in energy. Technically, the 4s orbital should almost come before it. So this hybridisation is highly unlikely.

Jan
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