A solution of an alkyl halide in dry ether reacts with magnesium to form alkyl magnesium halide.
RX + Mg in dry ether --> RMGX
RMgX compounds are known as Grignard reagents. They were discovered by Victor Grignard, a French National. He got the Nobel prize in Chemistry in 1912.
These compounds have many synthetic applications and are used in the preparation of many organic compounds.
Vinyl and aryl halides also form magnesium halides.