Imperial College London

Henry Rzepa

The period 1951--1954 was a golden one for structural chemistry;
[proteins](http://dx.doi.org/10.1073/pnas.37.4.205){target="_blank"},
[DNA](http://dx.doi.org/10.1038/171737a0){target="_blank"},
[Ferrocene](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3908){target="_blank"}
(1952) and the one I discuss here, a bonding model for Zeise's salt
(**3**).

![Zeise](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Zeise.svg){.aligncenter
.size-full .wp-image-10449 decoding="async"}

In "A review of π Complex Theory",  *Bull. Soc. Chim. Fr.*, **1951**, 1
8 , C79 (it is not online) M. J. S. Dewar sets out his theory of the
role of π-complexes in (mostly) organic chemistry. The paper derives
from an international colloquium held in Montpellier, in which audience
responses to the presentation are included as an annex to the article
itself. It is as a footnoted response (to P. Bartlett) that Dewar
presents his theory of the alkene-metal π-complex, of which the best
known example is [Zeise's
salt](http://en.wikipedia.org/wiki/Zeise's_salt){target="_blank"}
(**3**).

![Dewar-symmetry](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Dewar-symmetry.jpg){.aligncenter
.wp-image-10452 decoding="async"
width="350"}![Dewar1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Dewar1.jpeg){.aligncenter
.wp-image-10453 decoding="async" width="350"}

This diagram illustrates the binding of a silver cation Ag^+^ to ethene
(**1**). Dewar uses group theory to show how the molecular orbitals from
ethene can be combined with the atomic orbitals on the metal. Two filled
and two empty orbitals combine to give two new combinations, with a
total occupancy of four electrons defining the interaction between
alkene and metal. Dewar regards this four-electron-three-centre
interaction as distinctive from simply the formation of two single
metal-C bonds (a metallacyclopropane).

Zeise's salt itself derives from Pt^2+^ by addition of three chloride
anions to give PtCl~3~^--^. To compare this with Dewar's Ag^+^ example,
I use here just the naked metal cations **1-2**.^‡^ I went about this
analysis as follows:

1.  [I did ωB97XD/Dev2-SVP calculations, optimising the geometry
    into [C~2v~
    symmetry](http://www.webqc.org/symmetrypointgroup-c2v.html){target="_blank"}.]{style="line-height: 13px;"}
2.  The electronic configuration of Ag^+^ is \[Kr\].4d^10^.5s^0^ and
    Pt^2+^ is \[Xe\].4f^14^.5d^8^.6s^0^.
3.  The two metals therefore do differ; Ag^+^ can only accept electrons
    into a 5s atomic orbital (AO), whilst the Pt^2+^ can accept
    electrons into either the 6s or the empty 5d AO.
4.  The molecular orbitals identified for discussion here at *17*, *13*
    and *11 (*this is a pseudopotential calculation*)* of which *17* is
    doubly occupied for Ag^+^  and unoccupied for Pt^2+^. Why three when
    Dewar's analysis above describes only two? All (might) become clear
    shortly!
5.  Firstly, I start with the "back-bonding" orbital as shown on the
    right in Dewar's diagram. This is the interaction of the filled
    metal d~xz~ orbital with the alkene π\* empty anti-bonding orbital
    and the combination emerges as orbital *13* of the three considered
    here. It is *antisymmetric* with respect to rotation about the axis
    of symmetry and one of the two planes of symmetry, and is given the
    label (irreducible representation) **B**~1~. Map Dewar's "**-"**
    sign to [blue]{style="color: #00ccff;"} and "**+"** sign to
    [purple]{style="color: #ff00ff;"} to match them up. But also notice
    that the Pt orbital is rather more anti-bonding in the C-C region
    than Ag analogue. The C-C computed length (1.423Å) is indeed longer
    than that for the Ag complex (1.363Å, click on the images below to
    see a rotatable model of these orbitals). You will also notice that
    this orbital is "contaminated" with contributions from the C-H
    bonds; no longer are the π- and σ- electrons orthogonal as they are
    in ethene itself. This mixing of components from other parts of the
    molecules is what makes a clear-cut analysis of such systems
    trickier than you would infer by looking at Dewar's diagram above!
    This also happens from the ligands on the metal (Cl in Zeise's salt
    for example).  
    |                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                |
    |------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
    | Ag                                                                                                                                                                                                                                                                                                                                                                                                                           | Pt                                                                                                                                                                                                                                                                                                                                                                                                                             |
    | ![Ag-13](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Ag-13.jpeg){.aligncenter .size-full .wp-image-10454 decoding="async" onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/05/Ag_mo13.cub.xyz;zoom 80;isosurface color purple blue wp-content/uploads/2013/05/Ag_mo13.cub.jvxl translucent;');" width="210"} | ![Pt-13.](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Pt-13..jpeg){.aligncenter .size-full .wp-image-10455 decoding="async" onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/05/Pt_mo13.cub.xyz;zoom 80;isosurface color purple blue wp-content/uploads/2013/05/Pt_mo13.cub.jvxl translucent;');" width="210"} |
6.  Let us now go hunting from the second of Dewar's orbitals, which he
    describes as the interaction between the filled alkene π-MO and an
    empty Ag s-AO. Orbital *17* closely resembles Dewar's sketch on the
    left, although additional lobes can be seen. It is symmetric with
    respect to all three elements of symmetry (axis and two planes) and
    hence is labelled **A**~1~. Where Dewar writes that the two
    molecular bonds are distinct, he means that they have different
    symmetries and hence cannot interact with each other (they are
    orthogonal). But hang on; although this orbital is doubly occupied
    for Ag, it is unoccupied for Pt! So does that mean that Dewar's
    argument cannot hold for Zeise's salt itself (the bonding in this
    molecule is often referred to as the [Dewar-Chatt-Duncanson
    model](http://en.wikipedia.org/wiki/Dewar–Chatt–Duncanson_model){target="_blank"}\[cite\]10.1039%2FJR9530002939\[/cite\]).
    No. It turns out that for Ag, the alkene π-MO is interacting not
    with a pure unhybridised Ag s-AO, but with an *s+d~z~2* hybrid
    (albeit with rather more s and rather less *d~z~2).* This
    creates two modified hybrid AOs, one of which interacts with the
    alkene π-MO to give orbital *17*. This is what those extra lobes are
    about, the contribution from the 4d~z~2 AO on Ag. Because this
    combination on Pt is empty, the Ag complex has a shorter C-C bond
    than Pt.  
    |                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                               |
    |-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
    | Ag                                                                                                                                                                                                                                                                                                                                                                                                                            | Pt                                                                                                                                                                                                                                                                                                                                                                                                                            |
    | ![Ag-17](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Ag-17.jpeg){.aligncenter .size-full .wp-image-10467 decoding="async" onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/05/Ag_mo17.cub.xyz;zoom 80;isosurface color blue purple  wp-content/uploads/2013/05/Ag_mo17.cub.jvxl translucent;');" width="220"} | ![Pt-17](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Pt-17.jpeg){.aligncenter .size-full .wp-image-10466 decoding="async" onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/05/Pt_mo17.cub.xyz;zoom 80;isosurface color blue purple  wp-content/uploads/2013/05/Pt_mo17.cub.jvxl translucent;');" width="220"} |
7.  Well, Pt still needs explaining, since we have only found one of
    Dewar's two interactions. I mentioned that *s+d~z~2* hybrids could
    be created, and here the second of these interacts with the
    bonding alkene π-MO to give another **A**~1~ instance  *11,* again
    with the same symmetry properties with respect to the three elements
    of symmetry present (but this time with rather more *d~z~2 than s).*
    It is this orbital which is now occupied for Pt.  
    <table class="aligncenter" data-border="1" data-align="center">
    <colgroup>
    <col style="width: 50%" />
    <col style="width: 50%" />
    </colgroup>
    <tbody>
    <tr class="odd">
    <td>Ag</td>
    <td>Pt</td>
    </tr>
    <tr class="even">
    <td><img
    src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Ag-11.jpeg"
    class="aligncenter size-full wp-image-10474" decoding="async"
    onclick="jmolInitialize(&#39;../Jmol/&#39;,&#39;JmolAppletSigned.jar&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2013/05/Ag_mo11.cub.xyz;zoom 80;isosurface color purple blue wp-content/uploads/2013/05/Ag_mo11.cub.jvxl translucent;&#39;);"
    width="220" alt="Ag-11" /></td>
    <td><p><img
    src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/05/Ag-11.jpeg"
    class="aligncenter size-full wp-image-10474" decoding="async"
    onclick="jmolInitialize(&#39;../Jmol/&#39;,&#39;JmolAppletSigned.jar&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2013/05/Pt_mo11.cub.xyz;zoom 80;isosurface color purple blue wp-content/uploads/2013/05/Pt_mo11.cub.jvxl translucent;&#39;);"
    width="220" alt="Ag-11" /></p>
    <p> </p></td>
    </tr>
    </tbody>
    </table>

The famous Dewar π-complex model of alkene-metal interaction as applied
to the Ag^+^ cation describes one "normal molecular bond" and a second
bond "opposite in direction to the first", what we now call a back-bond.
What has emerged however is that two "normal molecular bonds" can be
identified for Ag^+^ based purely on their symmetry but only one for
Pt^2+^ (which of course has two valence electrons less) and both exhibit
one back-bond.  The diagram above must absorb a further pair of
electrons from a formally non-bonding filled *d~z~2* orbital, whilst
recognising that hybridisation may allow it too to take on some bonding
role.

You might ask what the missing orbitals *12*, *14-16* are?  Well,
formally they derive from the other occupied four metal d-orbitals, but
in fact mixed heavily with the C-H bonds of the ethene. I have to
conclude that a molecular orbital analysis of *e.g.* Zeise's salt (with
additional orbital mixing from the three chlorides) ends up being pretty
complex! But despite this complexity, Dewar's original hypothesis,
produced in response to a question from the audience, certainly started
something. It is worth reminding that the 1952 Nobel-prize winning
suggestion for the structure of
Ferrocene\[cite\]10.1021/ja01128a527\[/cite\] includes no group
theoretical orbital analysis of the bonding on a par with Dewar's 1951
insights.

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

^‡^ In fact, the MOs turn out to be pretty sensitive to the ligands
surrounding the metal, and so those presented here for the naked cations
will differ from those for "real molecules" such as Zeise's salt.

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

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


The period 1951–1954 was a golden one for structural chemistry; proteins, DNA, Ferrocene (1952) and the one I discuss here, a bonding model for Zeise’s salt (
<strong>
 3
</strong>
).    In “A review of π Complex Theory”,
<em>
 Bull. Soc. Chim. Fr.
</em>
,
<strong>
 1951
</strong>
, 1 8 , C79 (it is not online) M. J. S. Dewar sets out his theory of the role of π-complexes in (mostly) organic chemistry.

Messaggi di Rogue Scholar

The π-complex theory of metal-alkene compounds.