Lewis base

A Lewis base is any molecule or ion that can form a new coordinate covalent bond, by donating a pair of electrons, named after the American chemist Gilbert Lewis. The term base is ambiguous. This is one interpretation. To put it another way, any molecule with an electron lone pair in a bonding orbital may act as a Lewis base, as it is capable of accepting an ion with a single positive charge. Lewis bases are also generally capable of forming hydrogen bonds.

A nucleophile is a Lewis base. Lewis bases do not require a hydroxide ion as the electron acceptor. Some common examples include ammonia and amides. Many anions can also be considered Lewis bases such as the fluoride ion, F-.

When a Lewis acid and Lewis base form a complex ion the Lewis base is always the ligand.