coordinate covalent bond

Meaning: In an ordinary covalent bond, each atom contributes one electron to the shared pair. In a coordinate covalent (dative) bond, one atom supplies both electrons, usually as a lone pair, and the other atom has an empty orbital to receive them. The donor is the Lewis base and the acceptor is the Lewis acid. Examples: NH₃ + H⁺ → NH₄⁺, H₂O + H⁺ → H₃O⁺, BF₃ + NH₃ → F₃B–NH₃, and a metal ion binding ligands in a complex ion such as Cu(NH₃)₄²⁺.

On the MCAT: Expect it in Lewis acid-base questions, where you identify which species donates the lone pair and which accepts it. It also appears with complex ions, where a metal cation (acceptor) binds ligands (donors), and with formal charge in Lewis structures. A passage may use complex formation to explain why a sparingly soluble salt dissolves when a ligand is added, which you can predict with Le Châtelier's principle. Nucleophile-electrophile bond formation in organic chemistry follows the same logic.

Common confusion: Students treat a coordinate bond as a weaker or special kind of bond, or as ionic. Only the source of the electrons differs. In NH₄⁺, all four N–H bonds are equivalent in length and strength, and you cannot tell which one was the coordinate bond.