Illustrated Glossary of Organic Chemistry

Transmetalation: A reaction or mechanism step in which a metal bonded to another atom (usually a carbon atom) is exchanged for another metal.

Reaction of methyllithium (CH3Li; an organometallic compound having a carbon-metal bond) with CuI causes transmetalation: the carbon-lithium bond is exchanged for a carbon-copper bond. This forms methylcopper (CH3Cu), an organocopper compound. Reaction of CH3Cu with a second molecule of CH3Li causes a second transmetalation, this time forming lithium dimethylcuprate, (CH3)3CuLi.