ligand
Summary
A ligand is an ion or molecule that donates a lone pair to a central metal ion, forming a coordinate covalent bond in a complex ion. The ligand is the Lewis base and the metal is the Lewis acid. Adding a ligand that forms a stable complex often raises a metal salt's solubility.
Detail
Meaning: A ligand has at least one lone pair that it donates to a central metal ion, usually a transition metal. Both bonding electrons come from the ligand, so the bond is a coordinate covalent bond. Common ligands are H₂O, NH₃, Cl⁻, CN⁻ and OH⁻. Monodentate ligands bind through one donor atom. Bidentate ligands (ethylenediamine) and polydentate ligands (EDTA) bind through two or more, and these chelates generally form more stable complexes. The coordination number is the number of donor atoms bonded to the metal. In biochemistry, a ligand is any molecule that binds a protein or receptor.
Equation: M + nL ⇌ MLₙ, with Kf = [MLₙ]/([M][L]ⁿ). M is the free metal ion, L the ligand, n the number of ligands bound, and concentrations are in mol/L. Kf is the formation constant, with units of (mol/L)⁻ⁿ. A large Kf means a stable complex.
On the MCAT: Passages use complexes such as Cu(NH₃)₄²⁺ or Ag(NH₃)₂⁺. You may be asked to identify the Lewis acid and base, find the coordination number, or find the overall charge (metal charge plus ligand charges). A common case is a sparingly soluble salt like AgCl that dissolves when NH₃ is added. Complex formation removes free Ag⁺, so by Le Châtelier's principle the dissolution equilibrium shifts right.
Common confusion: The ligand is the electron-pair donor, not the metal. Coordination number counts bonds, not ligands: [Co(en)₃]³⁺ has three ligands but coordination number 6.
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.