The Grignard reaction is encountered early on in most chemistry courses, and most labs include the preparation of this reagent, typically by the following reaction:
2PhBr + 2Mg → 2PhMgBr ↔ MgBr2 + Ph2Mg
The reagent itself exists as part of an equilibrium, named after Schlenk, in which a significant concentration of a dialkyl or diarylmagnesium species is formed. The topic of this blog entry is to analyse the structure and bonding in this latter species.
First, the structure is shown below (for 2,6-diethylphenyl magnesium). This reveals a dimeric structure with a four membered ring core, comprising two Mg atoms connected by two bridging aryl groups.

The crystal structure of a di-aryl magnesium. Click to view 3D

A Simple bonding representation in Ph2Mg dimer
Is this really the best way of depicting the bonding in this species? A more subtle analysis of the bonding can be achieved using a technique known as ELF (involving analysis of the electron localization function). This reveals bonds as so-called synaptic basins, which come in two varieties; disynaptic basins corresponding to two-centre bonds, and trisynaptic basins which reveal three-centre bonds (there is also a monosynaptic basin which corresponds to electron lone pairs). Such an ELF analysis (based on a B3LYP/6-311G(d,p) computed wavefunction for Ph2Mg dimer) is shown below;

ELF analysis of the bonding in Ph2Mg dimer. Click for 3D model
It is interesting how a veritable mainstay of most taught chemistry courses, the Grignard reagent, can have such subtle aspects of the bonding surrounding both the metal atom and the aromatic groups, and how rarely this bonding is actually dissected in most text books.
Tags: Hypervalency, Interesting chemistry, metal, metal atom, metal centres, Mg atom
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