Imperial College London

Henry Rzepa

Every once in a while, one encounters a molecule which instantly makes
an interesting point. Thus Ruthenium is ten electrons short of
completing an [18-electron
shell](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3908){target="_blank"},
and it can form a complex with benzene on one face and a ligand known as
***trimethylenemethane*** on the
other\[cite\]10.1039/C39910001457\[/cite\].

![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/Ru.svg "Ru"){.aligncenter
.size-full .wp-image-8049 decoding="async"}

This four-carbon molecule has been known for a long
time\[cite\]10.1021/ar50055a003\[/cite\] as a highly reactive
intermediate, and as with cyclobutadiene (another 4π-electron species),
it can be greatly stabilised (and crystallised) by coordination to a
metal. Some twenty examples are known, and one is shown below.

::: {#attachment_8051 .wp-caption .aligncenter style="width: 262px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/JODLIX.jpg "JODLIX"){.wp-image-8051
loading="lazy" decoding="async"
aria-describedby="caption-attachment-8051"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/10/JODLIX.mol2;zoom 100;measure 1 3;measure 1 2;');"
width="252" height="268"}

JODLIX. Click for 3D.
:::

Why might it be interesting? Because the trimethylenemethane has four
carbons, of which the closest to the metal is the central one (2.03Å).
The three outer distances are longer at 2.19Å. If one were to (formally)
draw a bond from the metal to the central carbon atom, that atom would
become what is known as hemispherical, i.e. all four ligands would be
contained in a single hemisphere. This molecule is however normally
represented with bonds only to the peripheral carbons, and with [no
bond](http://www.ch.imperial.ac.uk/rzepa/blog/?p=2251){target="_blank"}
to what is after all the shortest of the four distances! 

A [QTAIM analysis](http://hdl.handle.net/10042/24184){target="_blank"}
gives the same answer. Only three bond-critical points are found in the
topology of the electron density (red), and at the centre where one
might have expected a Ru-C bond, one finds in fact a cage-critical point
(blue). Of course, another way of looking at the molecule is that the
trimethylenemethane simply contributes four electrons to the valence
shell of the metal without trying to partition these into bonds. These
four electrons, together with six from the benzene ligand, complete the
18-electron ruthenium shell.

![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/ru1.jpg "ru"){.aligncenter
.wp-image-8061 loading="lazy" decoding="async" width="296" height="303"}

So the take home message is that whilst the concept of discrete
two-centre bonds still has its uses, this little molecule reminds us
that bonds can be slippery customers. The common practice in most
computer codes that represent molecules, of joining up the shortest
contacts to form "bonds", would lead us in this instance to
hemispherical carbon. The consensus seems to be that this molecule does
not exhibit this.

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

**POSTSCRIPT:**  An  ELF analysis of the related
[Iron complex](http://hdl.handle.net/10042/24185){target="_blank"} is
shown below. It too shows three disynaptic basins for the three
peripheral  C-Fe "bonds" (red arrows), and none for the central one. The
total basin integration for these three is  4.82e, a bit more than the
nominal  4-electron ability of the ligand. 

::: {#attachment_8066 .wp-caption .aligncenter style="width: 293px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/10/Fe.jpg "Fe"){.wp-image-8066
loading="lazy" decoding="async"
aria-describedby="caption-attachment-8066"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2012/10/Fe_bas.mol2;');"
width="283" height="278"}

Click for 3D
:::

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


Every once in a while, one encounters a molecule which instantly makes an interesting point.

Messaggi di Rogue Scholar

Trimethylenemethane Ruthenium benzene