Imperial College London

Henry Rzepa

The following is a short question in a problem sheet associated with
introductory organic chemistry.

- **Q**: "*Show curly arrows for the formation of the product of the
  following reaction, together with a Lewis representation of that
  product: Et~2~O + MgBr~2~*".
- **A**: *Et~2~O^+^-Mg^--^Br~2~* (a product by the way that is known as
  magnesium bromide ethyl etherate, and which is commercially available
  as a solution).

First a few tutor-like comments. The Mg is tri-coordinate in this simple
representation, and if we assume that the bonds are covalent, has six
electrons in the Mg valence shell. In modern notation, the Mg has a
formal charge of -1 and the oxygen +1. The Mg thus does not have a
filled 3s/3p valence shell, which would be eight. But few (students or
tutors) go on to apply a reality check. So here is one.

The reality check involves a search for a crystal structure, which is
really trivial to set up. And what we find are the following.

1.  The first hit with exactly this stoichiometry has the CCDC
    code **TOQKIT** and a polymeric structure as below. Each Mg is
    coordinated by four (bridged) bromines and one oxygen, giving
    *trigonal bipyramidal* penta-coordination. The valence electron
    count at Mg is now eight, but distributed around five bonds, not
    four. Since we no longer have formal Lewis two-electron covalent
    bonds, it is difficult to assign a Lewis-like charge to the
    atoms.   

    ::: {#attachment_11541 .wp-caption .aligncenter style="width: 317px"}
    ![Click for
    3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/11/TOQKIT.jpg){.wp-image-11541
    loading="lazy" decoding="async"
    aria-describedby="caption-attachment-11541"
    onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/11/TOQKIT.mol;');"
    width="307" height="206"}
    Click for 3D
    :::
2.  The next hit actually corresponds to the stoichiometry 2R~2~O +
    MgBr~2~ (R=thf). This again is polymeric, but differs from the first
    structure in having *octahedral* Mg (six coordination).  

    ::: {#attachment_11544 .wp-caption .aligncenter style="width: 303px"}
    ![Click for
    3D.](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/11/MGBEET.jpg){.wp-image-11544
    loading="lazy" decoding="async"
    aria-describedby="caption-attachment-11544"
    onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/11/MGBEET.mol2;');"
    width="293" height="290"}
    Click for 3D.
    :::
3.  OK, even more ether: 4R~2~O + MgBr~2~. Finally, non polymeric but
    again with six-coordinate *octahedral* Mg. The Mg again has a filled
    valence octet, and again the bonds are not two-electron ones, hence
    no charges are attempted. So just a change in the stoichiometry can
    result in fascinating changes to the resulting structure.  

    ::: {#attachment_11545 .wp-caption .aligncenter style="width: 317px"}
    ![Click for
    3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/11/ZZZ.jpg){.wp-image-11545
    loading="lazy" decoding="async"
    aria-describedby="caption-attachment-11545"
    onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/11/ZZZVBQ.mol2;');"
    width="307" height="302"}
    Click for 3D
    :::
4.  Finally, a variation; benzyl magnesium bromide (a Grignard reagent)
    shows *tetrahedral* coordination.

::: {#attachment_11557 .wp-caption .aligncenter style="width: 312px"}
![Click for
3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/11/XECTOQ.jpg){.wp-image-11557
loading="lazy" decoding="async"
aria-describedby="caption-attachment-11557"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([450,450],'load wp-content/uploads/2013/11/XECTOQ.mol2;');"
width="302" height="233"}

Click for 3D
:::

Students (and tutors) who get as far as this are amply rewarded I hope!

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


The following is a short question in a problem sheet associated with introductory organic chemistry.

Messaggi di Rogue Scholar

Multiple personalities of Magnesium.