3

Structural data of sulphur dioxide
Figure 1: Structural data of $\ce{SO2}$. Taken from Wikimedia, where a full list of authors is available.

As can be seen from the structural data, the bond angle of $\ce{SO2}$ is almost precisely $120°$, meaning that the σ-orbitals can be described as almost perfect $\mathrm{sp^2}$ orbitals. This means that the sulphur lone pair is also in an $\mathrm{sp^2}$-like orbital while the fourth orbital of sulphur is of p-type to build a π-system not unlike that of the allyl anion. If sulphur were in the second period, this structure would not be surprising and indeed that of ozone is very similar (see figure 2).

Structural data of ozone
Figure 2: structural data of $\ce{O3}$. Taken from Wikimedia, where a full list of authors is available.

However, atoms of the third and higher periods generally have bond angles which are much closer to $90°$ as can be seen in figure 3 with the structure of $\ce{SF2}$. Typically, this is explained by their greater size which results in larger bond lengths and smaller bond angles, allowing for increased s-character of the remaining lone pairs. I myself have argued in this manner in previous answers. This should suggest an $\ce{SO2}$ bond angle much closer to $90°$, maybe around $100°$ degree as we indeed observe for $\ce{SF2}$ (see figure 3).

Structure of sulphur difluoride
Figure 3: Structural data of $\ce{SF2}$. Taken from Wikimedia, where a full list of authors is available.

But sulphur dioxide’s bond angle is $119°$ as stated above. Why does sulphur adopt such a large bond angle? Which factors disfavour the approximately $90°$ arrangement?

Jan
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  • Ah, Hero Of Time. Nice:D – andselisk Dec 17 '19 at 07:10
  • If you draw the Lewis structures of SF2 and SO2, you notice that in SO2 there is one lone pair on S, while in SF2 there are two lone pairs on S (this is indeed confirmed by NBO analisys on the geometries, which are predicted to be, at the b3lyp/cc-pvqz level, with angles of 118.7 and 98.6 degrees). The geometries which you would expect from VSEPR are those with 120 degrees for SO2 and 90<x<120 for SF2, which indeed agrees with the observation. The thing I would care about, if I had to find something strange, is the reason behind the deviation from the ideal 109 degrees in the case of SF2. –  Dec 23 '19 at 08:08
  • @The_Vinz the ideal for some that are not in the second period is no hybridisation and bond angles of approximately $90^\circ$. Even second period atoms would prefer to get there but they are too small so they have to extend the bonding angle. $\ce{AB4}$-systems are the exception because the tetrahedron is favoured by symmetry. – Jan Dec 23 '19 at 09:52

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