chelation
Summary
Chelation is the binding of one metal ion by a single ligand through two or more donor atoms, forming a ring. Chelates are usually more stable than complexes of comparable single-donor ligands, mainly because of a favorable entropy gain (the chelate effect).
Detail
Meaning: The metal ion is a Lewis acid and each donor atom on the ligand is a Lewis base, so the bonds are coordinate covalent. Ethylenediamine (bidentate), EDTA (hexadentate) and the porphyrin ring of heme binding Fe²⁺ are standard examples. When one polydentate ligand displaces several monodentate ligands such as water or NH₃, the number of free particles rises. ΔS is positive, ΔG becomes more negative, and the chelate is favored. This is the chelate effect.
Equation: M + L ⇌ ML, Kf = [ML]/[M][L], and ΔG° = −RT ln Kf = ΔH° − TΔS°. Kf is the formation constant (M⁻¹, often reported unitless), R is 8.314 J/(mol·K), T is temperature in K, ΔG°, ΔH° are in J/mol, and ΔS° is in J/(mol·K).
On the MCAT: A passage adds EDTA to a solution or enzyme assay to remove Mg²⁺, Ca²⁺ or Zn²⁺. Predict that free metal concentration falls and metal-dependent activity drops. Another version compares Kf of a chelate with that of a monodentate complex and asks you to explain it by entropy. Coordination number counts donor atoms, so [Co(en)₃]³⁺ has six.
Common confusion: The chelate effect is mainly entropic, not the result of stronger individual metal-donor bonds. A ligand that bridges two different metal ions is not chelating.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.