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.
- 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.
- 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).
- 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.
- Finally, a variation; benzyl magnesium bromide (a Grignard reagent) shows tetrahedral coordination.
Students (and tutors) who get as far as this are amply rewarded I hope!