Posts Tagged ‘tetrahedral’

Multiple personalities of Magnesium.

Tuesday, November 5th, 2013

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: Et2O + MgBr2“.
  • A: Et2O+-Mg–Br2 (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. 
    Click for 3D

    Click for 3D

  2. The next hit actually corresponds to the stoichiometry 2R2O + MgBr2 (R=thf). This again is polymeric, but differs from the first structure in having octahedral Mg (six coordination).
    Click for 3D.

    Click for 3D.

  3. OK, even more ether: 4R2O + MgBr2. 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.
    Click for 3D

    Click for 3D

  4. Finally, a variation; benzyl magnesium bromide (a Grignard reagent) shows tetrahedral coordination.
Click for 3D

Click for 3D

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

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Molecular toys: Tetrahedral cavities

Saturday, July 4th, 2009


An earlier post described how a (spherical) halide anion fitted snugly into a cavity generated by the simple molecule propanone, itself assembled by sodium cations coordinating to the oxygen. A recent elaboration of this theme, reminiscent of the children’s toys where objects have to be fitted into the only cavity that matches their shape, Nitschke and co-workers report the creation of a molecule with a tetrahedral rather than a spherical cavity (DOI: 10.1126/science.1175313 ), into which another but much smaller tetrahedral molecule is fitted.  The small molecule is P4, in which each of the three valencies of the P atom is directed to a corner of the tetrahedron. The large molecule  comprises four Fe atoms. These are each octahedrally coordinated with six ligand sites, three of which mimic the P atoms in also being directed towards the remaining three vertices of a tetrahedron.

P4 inside a Tetrahedral cavity.

Needless to say, the properties of the P4 molecule when entrained into this larger container are nothing like that of the free molecule. Now it is quite inert, but this is due purely to the snug fit. For example, the normal reaction of this molecule is to oxidize in air. But such oxidation would now produce a molecule too big to fit into the cavity. Hence no reaction!

So, now the search is on for a cubic container to include a cubic molecule!

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Posted in Interesting chemistry | 1 Comment »