Imperial College London

Henry Rzepa

The [post on
applying](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10937){target="_blank"}
VSEPR (\"valence shell electron pair repulsion\") theory to the geometry
of ClF~3~ has proved perennially popular. So here is a follow-up on
another little
molecue, [F~3~SN](https://winter.group.shef.ac.uk/vsepr/SF3N.html){target="_blank"}.
As the name implies, it is often represented with an S≡N bond. Here I
take a look at the conventional analysis.

[![trifluoorothionitrile](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/01/F3SN.svg){.aligncenter
.size-full .wp-image-14967 decoding="async" style="text-align: justify;"
width="100"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/01/F3SN.svg)

[This](https://winter.group.shef.ac.uk/vsepr/SF3N.html) is as follows:

1.  Six valence electrons on the central S atom.
2.  Three F atoms contribute one electron each.
3.  One electron from the N σ-bond.
4.  Donate two electrons from S to the two π-bonds.
5.  Eight electrons left around central S, ≡ four valence shell electron
    pairs.
6.  Hence a **tetrahedral** geometry.
7.  The bond-bond repulsions however are not all equal. The SN
    bond repels the three SF bonds more than the S-F bonds repel
    each-other.
8.  Hence the N-S-F angle is greater than the F-S-F angle, a distorted
    tetrahedron.

Now for a calculation\[cite\]10.14469/ch/191808\[/cite\];
 ωB97XD/Def2-TZVP, where the wavefunction is analysed using ELF
(electron localisation function), which is a useful way of locating the
centroids of bonds and lone pairs (click on diagram below to see 3D
model).

![Trifluorosulfonitrile](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/01/F3SN.jpg){.aligncenter
.size-full .wp-image-14967 decoding="async"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2016/01/elf09_F3SN.mol;spin 3;');"
style="text-align: justify;" width="300"}

- At the outset one notes that there are **six** ELF disynaptic basins
  surrounding the central S, integrating to a total of 7.05e. The sulfur
  is **NOT** hypervalent; it does not exceed the octet rule.
- These six \"*electron sub-pair*\" basins are arranged **octahedrally**
  around the sulfur. The coordination is NOT tetrahedral, as implied
  above.
- The three S-N basins have slightly more electrons (1.25e) than the
  three S-F basins (1.10e), resulting in ...
- the angle subtended at the S for the SN basins being 96° (a bit larger
  than octahedral) whilst the angle subtended at the S for the SF basins
  being smaller (89.9°). This matches point **7** above, but is achieved
  in an entirely different manner.
- As a result, the N-S-F angle (122.5°) is larger than the ideal
  tetrahedral angle and the F-S-F angle (93.9°) is smaller, an
  alternative way of expressing point **7** above.
- The S≡N triple bond as shown above does have some reality;  it is a
  \"*banana bond*\" with three connectors rather than two. Each banana
  bond however has only 1.25e, so the bond order of this motif is
  \~four (not six) but nevertheless resulting in a short S-N distance
  (1.406Å) with multiple character.

So we have achieved the same result as classical VSEPR, but using
partial rather than full electron pairs to do so. We got the same result
with ClF~3~ before. So perhaps this variation could be called \"*valence
shell partial electron pair repulsions*\" or **VSPEPR**.

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


The post on applying VSEPR ("valence shell electron pair repulsion") theory to the geometry of ClF
<sub>
 3
</sub>
has proved perennially popular. So here is a follow-up on another little molecue, F
<sub>
 3
</sub>
SN. As the name implies, it is often represented with an S≡N bond. Here I take a look at the conventional analysis. This is as follows:         Six valence electrons on the central S atom.

Rogue Scholar Beiträge

VSEPR Theory: A closer look at trifluorothionitrile, NSF3.