reactivity - Grignards do transmetallation?

Grignards don't add to alkyl halides to create linear carbon extensions because Grignards do transmetallation rather than SN2 displacement of halide.

Is this correct?

I thought transmetallation a) involved two metals and b) involved ligand exchange between the two metals … correct me if I'm wrong, but if I have a Grignard and an alkyl halide, I only have one metal.

Answer

Prof told me Grignards don't add to alkyl halides

Grignards can react with alkyl halides to form the expected coupling product. However yields are usually very low except under special conditions. Improved yields (but yields that would still be considered poor for a synthetic route) result when the halide involved is a good leaving group (I ~ Br > Cl), or when the Grignard is made from a stabilized "R" group such as allyl or benzyl.

Here is an interesting example where the reaction of an aryl Grignard reagent with an alkyl halide was examined.

"When a THF solution of p-tolylmagnesium bromide 1a and an equimolar amount of 1-iodoheptane under nitrogen was stirred for 48 h at room temperature, only a very small amount (7% by GC) of the expected coupling product, 1-p-tolylheptane, was formed."

So in principle, Grignard reagents are capable of reactions that look like $\ce{S_{N}2}$ reactions (who knows what the mechanism is when a metal is involved), the yields are just very low.