Imperial College London

Henry Rzepa

I [noted
previously](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10870 "The butterfly effect in chemistry: aromaticity on the edge of chaos."){target="_blank"
rel="noopener"} that some 8-ring cyclic compounds could exist in either
a planar-aromatic or a non-planar-non-aromatic mode, the mode being
determined by apparently quite small changes in a ring substituent.
Hunting for other examples of such chemistry on the edge, I did a search
of the Cambridge crystal database for metal sulfides. 

The search was specified as following:

1.  [Any element from one of the three transition metal series, TS1, TS2
    or TS3]{style="line-height: 13px;"}
2.  to contain a M-S bond (any type of bond)
3.  with the restriction that the sulfur has only one atom attached (the
    metal)
4.  R \< 0.05
5.  No errors and no disorder

The results for the three transition series were quite different. The
first row indicated a distribution with a single maximum (\~2.3Å),
albeit with a very long tail reaching out to 3.2Å. The second row had a
very clear bimodel distribution, with two peaks, at \~2.2Å and \~2.55Å,
the latter having the greater number of examples. The third row showed
an inverse distribution, with the prominent peak at \~2.2Å and the
smaller peak at 2.55Å. 

![TS1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/TS1.jpg){.aligncenter
.size-full .wp-image-10885 decoding="async"
width="440"}![TS2](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/TS2.jpg){.aligncenter
.size-full .wp-image-10884 decoding="async" width="440"}
![TS3](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/TS3.jpg){.aligncenter
.size-full .wp-image-10886 decoding="async" width="440"}

A search for the possible reasons for the bimodal distributions is now
needed. This could be done in two ways; (a) to refine the search, or (b)
to individually scrutinize each example to identify if any chemical
reasons can be found. Since there are 100s of examples, I will
concentrate on the first tactic. The most obvious is that the compounds
cluster into neutral and ionic, as below, which allows the search to be
constrained to only entries where a negative charge has been assigned to
the sulfur.

![MS](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/MS.svg){.aligncenter
.size-full .wp-image-10892 decoding="async"}

![TS2m](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/TS2m.jpg){.aligncenter
.size-full .wp-image-10893 decoding="async" width="440"}
![TS3m](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/07/TS3m.jpg){.aligncenter
.size-full .wp-image-10894 decoding="async" width="440"}

This reveals such entries are a fraction of the total, and for these the
C-S bond length clusters around 2.2Å. Which leaves us with a mystery.
This would mean that the neutral systems, presumably with a formal M=S
bond, would have to be responsible for the entries at \~2.55Å. Could it
really be that a higher bond order results in a longer bond? 

What the search does not allow us to do (or I am unaware of how to do
it) is to identify whether there are any pair of examples where the
ligands surrounding M are identical but where one of the pair has a
short and the other a long M-S bond. Or indeed such a pair where the
differences between the ligands are small (however that is defined).
Similarly, one cannot constrain the searches by spin state to see if
that might be responsible. It would also be nice to have the system
automatically evaluate whether the valence shell on the metal is full
(18) or not (16, 17) to see if that is related to the bond length.
Indeed, an example of needing to count the valence shell was recently
brought to my attention by a student, and it will be the subject of a
future post. 

------------------------------------------------------------------------

#### Acknowledgments

This post has been cross-posted in PDF format at
[Authorea](https://doi.org/10.15200/winn.143118.82135){rel="noopener"
target="_blank"}.

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


I noted previously that some 8-ring cyclic compounds could exist in either a planar-aromatic or a non-planar-non-aromatic mode, the mode being determined by apparently quite small changes in a ring substituent. Hunting for other examples of such chemistry on the edge, I did a search of the Cambridge crystal database for metal sulfides.

Messaggi di Rogue Scholar

The butterfly effect in chemistry: Bimodal M~S bonds?