chelate

Meaning: Each donor atom gives a lone pair to the metal, so the ligand is a Lewis base and the metal ion is a Lewis acid. Ethylenediamine is bidentate, and EDTA is hexadentate. The chelate effect: one polydentate ligand displaces several monodentate ligands, so more free particles are released and entropy rises. In [Ni(H₂O)₆]²⁺ + 3 en ⇌ [Ni(en)₃]²⁺ + 6 H₂O, 4 particles become 7, which makes ΔG more negative.

Equation: Kf = [ML]/([M][L]) and ΔG° = −RT ln Kf. Kf is the formation constant (a larger value means a more stable complex), brackets are concentrations in mol/L, R = 8.314 J/(mol·K), T is in K, and ΔG° is in J/mol.

On the MCAT: Passages use EDTA to bind Ca²⁺ or Mg²⁺ in water-hardness titrations, to sequester toxic metal ions, or to stop metal-dependent reactions. Heme, in which a porphyrin ring binds Fe²⁺, is a biological example. You may be asked to predict which ligand binds more tightly (usually the polydentate one). You may also be asked what adding a chelator does to free metal ion concentration, which falls as the equilibrium shifts toward the complex.

Common confusion: Many ligands on one metal do not make a chelate. [Ni(H₂O)₆]²⁺ has six monodentate ligands. A chelate needs one ligand attached at two or more sites. Coordination number counts donor atoms, so one EDTA gives 6, not 1.