Imperial College London

Henry Rzepa

In the [preceding
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=10448 "The π-complex theory of metal-alkene compounds."){target="_blank"},
I introduced Dewar's π-complex theory for alkene-metal compounds,
outlining the molecular orbital analysis he presented, in which the
filled π-MO of the alkene donates into a Ag^+^ empty metal orbital and
back-donation occurs from a filled metal orbital into the alkene π\* MO.
Here I play a little "what if" game with this scenario to see what one
can learn from doing so.

![Au+cbd](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Au+cbd.svg){.aligncenter
.size-full .wp-image-10502 decoding="async"}

Firstly, I will use Au^+^ instead of Ag^+^, so as to make a comparison
with Pt^2+^ a little more direct. The electronic configurations are of
course \[Xe\].4f^14^.5d^10^.6s^0^ and \[Xe\].4f^14^.5d^8^.6s^0^
respectively. I will also replace a simple ethene with cyclobutadiene,
the intent here being that this cyclo-diene is a very much
better π-donor due to its anti-aromatic character. It also now has the
possibility of acting as a four or a two-electron donor. I started with
M=Pt^+^\[cite\]10.6084/m9.figshare.703546\[/cite\] by adding another
double bond to the structure of the ethene complex. 

![Pt-cbd](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Pt-cbd.jpeg){.aligncenter
.wp-image-10503 loading="lazy" decoding="async" width="312"
height="293"}

Optimising this starting structure in fact moves the metal and the final
geometry has C~4v~ symmetry; in other words the metal is bound
symmetrically to all four carbons. The four C-C lengths are all the same
(1.46Å) and strongly suggest that four electrons from the cyclobutadiene
are participating in bonding; the Pt^2+^ is clearly capable of accepting
four electrons, two into 6s^0 ^and two into 5d^8^. In the process, the
cyclobutadiene looses its antiaromaticity. The molecular orbitals of
this species are all lovely; I illustrate just one below.

::: {#attachment_10505 .wp-caption .aligncenter style="width: 303px"}
![Click for
3D.](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Pt-cbd-21.jpeg){.wp-image-10505
loading="lazy" decoding="async"
aria-describedby="caption-attachment-10505"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/05/Pt4_mo21.cub.xyz;zoom 80;isosurface color green blue wp-content/uploads/2013/05/Pt4_mo21.cub.jvxl translucent;');"
width="293" height="293"}

Click for 3D.
:::

If the Pt in this C~4v~ structure is mutated into Au^+^, the resulting
optimised stationary point exhibits a negative force constant
characteristic of a transition
state\[cite\]10.6084/m9.figshare.703547\[/cite\]. As the d-shell is
already fully, the Au can only accept two electrons, and this is
therefore a nice illustration of the "18-electron" rule in operation.
So, the Au^+^ complex must exist in at least one lower energy form. For
example, one where the Au^+^ is coordinated to only one alkene is 94
kcal/mol lower in free
energy.\[cite\]10.6084/m9.figshare.703576\[/cite\] This form results in
electrons from the coordinated alkene being donated into the 6s Au
orbital, and this action reduces the anti-aromaticity of the
cyclobutadiene ring.

![Au-cs](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Au-cs.jpg){.aligncenter
.wp-image-10508 loading="lazy" decoding="async" width="210"
height="169"}

Another isomer also achieves this result, resulting in a further
lowering in free energy of 11.0
kcal/mol\[cite\]10.6084/m9.figshare.703577\[/cite\] The anti-aromaticity
this time is eliminated by forming an allyl cation on the ring. I have
described this mode in [another
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=4952){target="_blank"},
commenting on the effect when a guanidinium cation interacts with
cyclobutadiene.![Au-cs1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Au-cs1.jpg){.aligncenter
.wp-image-10512 loading="lazy" decoding="async" width="169"
height="173"}

We have learnt that cyclobutadiene has many modes for eliminating
4n-electron antiaromaticity and other destabilising influences upon the
ring. It can accept four electrons from a suitable acceptor (Pt^2+^), or
two electrons from Au^+^ in two different ways.

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


In the preceding post, I introduced Dewar’s π-complex theory for alkene-metal compounds, outlining the molecular orbital analysis he presented, in which the filled π-MO of the alkene donates into a Ag
<sup>
 +
</sup>
empty metal orbital and back-donation occurs from a filled metal orbital into the alkene π* MO. Here I play a little “what if” game with this scenario to see what one can learn from doing so.    Firstly, I will use

Messaggi di Rogue Scholar

Au and Pt π-complexes of cyclobutadiene.