A nucleophile is a species which reacts by donating a pair of electrons to form a covalent bond. They can be identified as having a lone pair of electrons in thier outer atomic subshell which allows them to complete addition and sustitution reaction with the positive end of polar molecules.
Nucleophiles can be classified according to their overall charge. Neutral nucleophiles include ammonia (NH3), water (H2O) and carbon monoxide (CO). Negative nucleophiles include hydroxyl ions (OH-), chloride ions (Cl-) and cyanide ions (CN-).