Imperial College London

Henry Rzepa

A while ago, I
[explored](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10937){target="_blank"}
how the 3-coordinate halogen compound ClF~3~ is conventionally analyzed
using VSEPR (valence shell electron pair repulsion theory). Here I
(belatedly) look at other such tri-coordinate halogen compounds using
known structures gleaned from the crystal structure database (CSD).

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/7-SQ.jpg){decoding="async"
width="300"}

The search query specifies 7A as the central atom, defined with just
three bonded (non-metallic) atoms. Initially, if no constraint on any
cyclicity in the three 7A-NM bonds is made (and with R \< 0.1, no
errors, no disorder), the following result emerges.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/tr-coord-hal.jpg){decoding="async"
width="440"}

I have plotted the three angle variables using the X/Y axes above and
used colour to indicate the third angle (red = \~180°, blue = \~90°).
The clusters show that two of the angles are \~90° and only one
is \~180°. There is also a set of blue points (\~90°) which show a
linear correlation and which can be shown to derive from cyclicity,
as the plot below reveals when acyclicity is specified for all three
NM-7A bonds.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/tr-coord-hal-acyclic.jpg){decoding="async"
width="440"}

In this distribution, the two clusters for ANG1 or ANG2 of \~180° are
small and compact, but the cluster where both ANG1 and ANG2 are \~90° is
much more diffuse. Not all of the points in this cluster show as red
(ANG3 \~180°); there are a few cyan or blue examples here
too; indicating all three angles are in the range 140-90°. This result
is not arising from cyclic constraints. 

This wider look at 3-coordinate compounds in group 17 (the halogens)
quickly reveals a class of such molecules where all three angles are
relatively small. This suggests that a closer look at the bonding in
these systems, especially in terms of VSEPR, might be rewarding!

I end with an equivalent search for group 18 (the noble gases). Although
the number of examples is small, all show the two small/one large angle
so characteristic of chlorine trifluoride itself. 

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/06/tr-coord-ig.jpg){decoding="async"
width="440"}

The above is I think a good example of (big?) data mining, where one is
searching for patterns, and if lucky spotting patterns that deviate from
the norm to investigate the possibility of new chemical
phenomena.\[cite\]10.1021/acs.jchemed.5b00346\[/cite\] It is also
interesting to speculate upon the origins of why two of the clusters
shown above are small and compact and the third is much more diffuse.

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


A while ago, I explored how the 3-coordinate halogen compound ClF
<sub>
 3
</sub>
is conventionally analyzed using VSEPR (valence shell electron pair repulsion theory). Here I (belatedly) look at other such tri-coordinate halogen compounds using known structures gleaned from the crystal structure database (CSD).     	   	The search query specifies 7A as the central atom, defined with just three bonded (non-metallic) atoms.

Rogue Scholar Beiträge

A wider look at chlorine trifluoride: crystal structures and data mining.