Stability of Metal Complexes

The stability of a coordination complex can be described in two different ways. Thermodynamic stability refers to the free energy change (ΔG) of the complex-formation reaction -- how favourable it is for the complex to form in the first place. Kinetic stability instead refers to how readily the complex undergoes ligand substitution once formed: complexes that undergo rapid ligand substitution are called labile complexes, while those that substitute very slowly (or not at all) are called inert complexes. A complex's stability is quantified by its stability constant (β), a measure of its resistance to having one ligand displaced by another, reflecting the degree of association between the species at equilibrium. For the reaction Cu²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄]²⁺, the stability constant is β = [Cu(NH₃)₄²⁺] / ([Cu²⁺][NH₃]⁴); as the concentration of the complex ion at equilibrium increases (relative to the free ion and free ligand), the value of β increases, so the greater the stability constant, the greater the stability of the complex. Because coordination complexes in solution can dissociate to a small extent, and the extent of that dissociation depends on the strength of the metal-ligand bond (a stronger M←L bond means less dissociation), stability can equally be expressed through the reverse, dissociation equilibrium: for [Cu(NH₃)₄]²⁺ ⇌ Cu²⁺ + 4NH₃, the dissociation (instability) constant is α = [Cu²⁺][NH₃]⁴ / [Cu(NH₃)₄²⁺]. Since α and β describe the same equilibrium from opposite directio …