Nucleophilic center:

Nucleophiles comprise a particular atom or region of the molecule that is electron rich. This is termed as the nucleophilic center. An ion's nucleophilic center is the atom bearing a lone pair of the electrons and negative charge. The nucleophilic center of a neutral molecule is generally an atom along with a lone pair of electrons (for example nitro- gen or oxygen), or a multiple bond (for example alkene, alkyne, aromatic ring).