Organic Chemists refer to
carbon atoms as
nucleophilic if they have a partial negative charge on them, and thus have an
affinity for reacting with delta-positive carbon atoms such as in
carbonyls.
Grignard reagents of the form R-Mg-X, where a carbon atom is bonded to electron-donating magnesium, are prime examples of nucleophilic species. Grignards and other carbanions, being nucleophilic, are useful intermediates in organic synthesis, as they allow one to easily extend the carbon skeleton of a molecule.