While nucleophiles donate two electrons in exchange for forming two-electron covalent bonds with the electrophiles, the negative charge due to the two electrons is not obvious in many instances. Neutral molecules like Benzene, water, and ammonia can act as nucleophiles.
A broad way to categorize nucleophiles is as charged or neutral (uncharged).
1. Charged Species – If an atom gains electrons, the electron excess is shown with a negative charge; such species are anions. Some anions can act as nucleophiles, for example, Cl-, Br-, and I-. However, highly electronegative F- is not a strong nucleophile. It prefers to hold on to its electrons than donate.
The negative charge can also be acquired post a heterolytic bond cleavage, like in the case of R-, HO-, RO-, and NH2-, generating a nucleophile.
It is important to note that if a nucleophile generated is highly unstable, it will then be highly reactive and act as a Lewis base by picking up H+ as an electrophile, forming a Nuc-H bond.
Neutral molecules like BH3 AlH3 can also acquire the negative charge with a central atom with an empty p-orbital to form charged BH4-, AlH4- molecules. In such molecules, the BH3-—H or AlH3-—H sigma bond is the nucleophile.
2. Neutral species – Neutral molecules with lone pair of electrons, such as H2O, NH3, and an electron-rich pie system of Benzene, alkene, can act as a nucleophile.
The pie bonds’ donating capability, known as nucleophilicity, gets stronger when the pie bonds are next to a substituent or atom that helps push the pie bond electrons. The electron push aids in a better nucleophilic attack. The push occurs by donation (structure 1) or delocalization (structures 2, 3, and 4).