Metalation (metallation) is a chemical reaction which results in a metal atom being attached to a (usually organic) molecule.
Lithium metal in contact with an organohalide lithiates the organic molecule and gives the organolithium reagent and lithium halide. Magnesium similarly metalates organohalides to give Grignard reagents.
Protons on a molecule that are acidic, even only slightly, can be exchanged for a metal by a suitable butyllithium reagent mixture.
Transmetalation is the exchange of two metals between organic molecules. For example, transmetalations can be accomplished by reacting an organolithium reagent with a metal salt.