Imperial College London

Henry Rzepa

In the previous two posts, I noted the recent suggestion of how a stable
frozen S~N~2 transition state might be made. This is characterised by a
central carbon with five coordinated ligands. The original suggestion
included two astatine atoms as ligands (X=At), but in my post I
suggested an alternative which would have five carbon ligands instead
(X=cyclopentadienyl anion).

::: {#attachment_784 .wp-caption .aligncenter style="width: 257px"}
![The Sn2 transition
state](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/09/sn2.jpg "sn2"){.size-full
.wp-image-784 loading="lazy" decoding="async"
aria-describedby="caption-attachment-784" width="247" height="185"}

The Sn2 transition state
:::

However, these five ligands are not all equal; far from it. Three form
normal strength bonds to the central carbon, and two very weak
(deci)bonds. So, could a molecule be made with five equal bonds all
coordinated to a central carbon atom? Well, the inspiration for
designing such a molecule comes with the report of a remarkable compound
of silicon by Jutzi and
co-workers\[cite\]10.1126/science.1099879\[/cite\]. Examples with Ge, Sn
and  Pb are also known.

::: {#attachment_848 .wp-caption .aligncenter style="width: 246px"}
![The silyliumylidene
cation](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/09/Si.jpg "Si"){.size-full
.wp-image-848 loading="lazy" decoding="async"
aria-describedby="caption-attachment-848" width="236" height="188"}

The silyliumylidene cation
:::

Using a large non-coordinating anionic counterion, a crystal structure
could be determined for the pentamethyl derivative (Refcode: BIDLEG),
which reveals the five-fold symmetry of the silicon coordination. The
obvious mutation therefore is to see if the corresponding carbon
compound might be stable.  A B3LYP/6-311G(d,p) calculation (DOI:
[10.14469/ch/2408](https://doi.org/10.14469/ch/2408){target="references"})
run with  C~5~ symmetry reveals this system to have only positive force
constants, with five equal C-C bonds to the central carbon, each with
the unusual length of 1.799Å. The *bouncing castle* vibrational mode
involving the pentacoordinate carbon has a value of  767 cm^-1^

::: {#attachment_852 .wp-caption .aligncenter style="width: 250px"}
![The carbyliumylidene
Cation](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/09/C5.jpg "C5"){.size-full
.wp-image-852 loading="lazy" decoding="async"
aria-describedby="caption-attachment-852" width="240" height="245"}

The carbyliumylidene Cation
:::

So, not only do we now have a clearly penta-coordinate carbon, all five
bonds are of equal length! More unusual still, all five ligands occupy
one hemisphere of the carbon coordination. Why might such a geometry be
stable? Well, as with the silicon analogue, C^2+^ has only two valence
electrons left. To elevate this to the standard octet, it must accept
six electrons, and the cyclopentadienyl anion fulfils this role
perfectly. The top three occupied molecular orbitals are shown below.

<table data-border="0">
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<tbody>
<tr class="odd">
<td><div id="attachment_855" class="wp-caption aligncenter"
style="width: 190px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/09/C5-homo1.jpg"
title="C5-homo" class="size-full wp-image-855" loading="lazy"
decoding="async" aria-describedby="caption-attachment-855"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2009/09/C5-40.jvxl;isosurface &quot;&quot; translucent;zoom 120;spin 3;&#39;);"
width="180" height="221" alt="The HOMO orbital" />
<p>The HOMO orbital. Click for 3D</p>
</div></td>
<td><div id="attachment_854" class="wp-caption aligncenter"
style="width: 246px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/09/C5-homo-1.jpg"
title="C5-homo-1" class="size-full wp-image-854" loading="lazy"
decoding="async" aria-describedby="caption-attachment-854"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2009/09/C5-39.jvxl;isosurface &quot;&quot; translucent;zoom 120;spin 3;&#39;);"
width="236" height="211" alt="The HOMO-1 (degenerate) orbital" />
<p>The HOMO-1 (degenerate) orbital. Click for 3D</p>
</div></td>
</tr>
</tbody>
</table>

An AIM analysis (below) shows five equal bond critical points, with ρ(r)
0.13 au for each (see [previous
post](http://www.ch.ic.ac.uk/rzepa/blog/?p=811) for comparison), a value
which probably can be described by the term **bond**. The ∇^2^ρ value of
+0.07 au is similar to that quoted in the previous post. Noteworthy is
the observation that no ring critical point (RCP, yellow dots) can be
found for the cyclopentadienyl ring itself, only for the five
three-membered rings to the pentacoordinate atom.

::: {#attachment_859 .wp-caption .aligncenter style="width: 264px"}
![AIM (Atoms-in-Molecules)
analysis](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/09/C5-aim.jpg "C5-aim"){.size-full
.wp-image-859 loading="lazy" decoding="async"
aria-describedby="caption-attachment-859" width="254" height="244"}

AIM (Atoms-in-Molecules) analysis
:::

Can the species be made? Well, given that it seems the case that carbon
and silicon chemistries are inverted, ie what is stable with silicon is
unstable with carbon, and vice versa, the answer is probably no. But one
never knows until one has tried!

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


In the previous two posts, I noted the recent suggestion of how a stable frozen S
<sub>
 N
</sub>
2 transition state might be made. This is characterised by a central carbon with five coordinated ligands.

Postagens de Rogue Scholar

It’s penta-coordinate carbon Spock- but not as we know it!