lewis base

Meaning: A Lewis base supplies both electrons for a new bond. The Lewis acid accepts that pair into an empty orbital. The new bond is a coordinate covalent bond, and the product is called an adduct. Typical Lewis bases have a lone pair: NH₃, H₂O, OH⁻, Cl⁻, and the ligands in complex ions. Example: BF₃ + :NH₃ → F₃B–NH₃, where NH₃ donates and boron (an incomplete octet) accepts. In a complex ion, the metal cation is the Lewis acid and each ligand is a Lewis base.

On the MCAT: Passages often describe a metal ion dissolving in a ligand solution, such as Ag⁺ with NH₃, and ask you to name the acid and base or to explain why solubility rises (complex-ion formation, Kf). Other questions show a reaction and ask which reactant donates the electron pair. Follow the curved arrow: it starts at the lone pair of the Lewis base. In organic chemistry the same idea is called a nucleophile, which attacks an electron-poor atom.

Common confusion: Students think a base must accept H⁺ or release OH⁻. Those are the Brønsted and Arrhenius definitions. A Brønsted base is a Lewis base whose acceptor is H⁺, but the Lewis definition also covers reactions with no proton, like NH₃ with BF₃. "Donating electrons" here means sharing a pair in a bond, not the electron loss of oxidation.