Halogen/Metal exchange reaction, a mechanistical insight

Although the halogen/metal exchange reaction (R1-X + R2-M = R1-M + R2-X) is a cornerstone in the methodology of organometallic chemistry, its mechanism is far from being settled.

3 structures have been postulated as possible transition state/intermediate for this reaction: (1) A four-centres, rhomboid [R1MXR2] transition state, (2) Ate-complexes [(R1-X-R2)M], and (3) radicalary.

When R1 and R2 are such that the ate-complex can enjoy substantial thermodynamic stability, it can be a detectable intermediate. The structural and kinetic stability of the ate-complexes permits them to be present as solvent separated ion pairs (SSIP) or even as dissociated ions.
In this cases, C-X bond formation precedes the C-M bond formation, and the TS may be approximated by the contact ion pair (CIP), in which a linear dialkyl-halogenate anion is distorted by coulombic attraction between the metal cation and the negatively charged carbon atoms.

In addition, the preferred pathway for a quite stable ate-complex is not one based on a SET mechanism. However, ate complexes are highly sensitive to enter into a SET-inititiated radical processes (even a small amount of radical may trigger the SET pathway). In the competence cases, lowering the temperature can be a way to favour the polar mechanism, as can be the use of additives as Mg(OTf)2 (by adding a common ion).

For an indeeper discussion see Org.Lett.2003, 5, 313

Opening a broader window, and as a law of thumbs:

1. Aryl bromides and iodides react through an ate-complex
2. Primary alkyl iodides react via polar mechanism
3. Secondary alkyl iodides undergo polar/radical competition
4. Alkyl iodides react via radical mechanism
5. I-Mg exchange is via ate-complex

For a rationale with examples