A organometallic reagent usually consists of lithium,magnesium,sodium,potassium,calcium or aluminium.
The reason for this is that organometallic regents are usually nucleophiles. When a metal of high electropositivity is attatched to a carbon the electrons are highly polarised towards the carbon and so it becomes a very powerful nucleophile. The reason that only these metals can be used is that there is a high difference in electropositivity compared to carbon. It then follows that the best regent is one which involves lithium or magnesium
A organometallic is something which has a direct metal- carbon bond.
Usually organometallics used commercially are lithium and magnesium because they are highly polarized towards carbon due to a big difference in electronegativity and so are very powerful nucleophiles.
Author: Gillian Cooper (document modification date: 22nd May 2003)