coordinate covalent

Meaning: A coordinate covalent bond (dative bond) is a shared electron pair in which both electrons come from the same atom. The donor has a lone pair and acts as a Lewis base. The acceptor has an empty orbital and acts as a Lewis acid. Examples are NH₃ + H⁺ → NH₄⁺, BF₃ + NH₃ → F₃B–NH₃, and metal cations bonded to ligands such as NH₃, H₂O, Cl⁻ or CN⁻ in complex ions. After formation, the bond is the same as an ordinary covalent bond. In NH₄⁺ all four N–H bonds are equivalent.

Equation: Cu²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄]²⁺, with Kf = [[Cu(NH₃)₄]²⁺] / ([Cu²⁺][NH₃]⁴). Kf is the formation constant. Concentrations are in mol/L (M), and Kf is usually quoted without units.

On the MCAT: You may be asked to pick the donor and acceptor in a reaction or Lewis structure, or to classify a species as a Lewis acid or base. Passages on complex ions often show that adding a ligand raises the solubility of a sparingly soluble salt. The complex ties up free metal ion, and Le Châtelier's principle shifts the dissolution equilibrium forward.

Common confusion: Students treat a coordinate covalent bond as a weaker, ionic or different kind of bond. Only the origin of the electrons differs, and the bond itself is covalent. The donor must have a lone pair, so H⁺, which has no electrons, can only be an acceptor.