coordinate complex

Meaning: A central metal ion, often a transition metal, is bonded to ligands. Each ligand is a Lewis base that donates both electrons of a bond; the metal is the Lewis acid. The number of donor bonds at the metal is the coordination number, commonly 2, 4 or 6. Ligands such as NH₃, H₂O, Cl⁻ and CN⁻ are monodentate. Ethylenediamine and EDTA are bidentate or polydentate and form chelates.

Equation: Kf = [ML₄] / ([M][L]⁴), shown here for four ligands. In general, Kf = [MLn] / ([M][L]ⁿ). [MLn] is the complex, [M] the free metal ion, [L] the free ligand (all in mol/L), and n the number of ligands. Kf is usually quoted without units. Charge of complex = metal charge + sum of ligand charges.

On the MCAT: A passage adds NH₃ or CN⁻ to a sparingly soluble salt such as AgCl. The complex Ag(NH₃)₂⁺ forms, free Ag⁺ drops, and Le Chatelier's principle shifts dissolution forward, so solubility rises. You may also compare complex stability by Kf, find a complex's charge, or label the Lewis acid and base. Heme iron binding O₂ is a biology example.

Common confusion: The metal is the electron acceptor, not the donor, and the bond is a coordinate covalent bond, not simply ionic. Coordination number counts bonds, not ligands: [Co(en)₃]³⁺ has three ligands but a coordination number of 6.