Stability of co ordination complexes

An INORGANIC CHEMISTRY tOPIC.

1. Stability of Co-ordination Complexes Oresented By: Dr. Shipra Sgar PHD: Inorganic Chemistry

2. Stability of Co-ordination Complexes The stability of Co-ordination complexes is determined to be its existence in aqueous solution w.r.t its bond dissociation energy, Gibbs free energy ( Δ G ), standard electrode potential (E ) or pH of the solution . Stability of complexes is also depend on the activation Energy (E A ) or speed of the substitution reaction. ( i ) Thermodynamic Stability (ii) Kinetic Stability Thermodynamic stability measure in term of complex to exist in equilibrium condition or its measure a tendency of a metal ion to form a particular complex ion. M-L bond energy Stability constant or formation constant ( K f ) is directly related to Δ G Δ G reaction = Δ G product - Δ G reactant Δ G = - RTlnK f Δ G = -nFE RTlnK f = nFE Kf α E

3. Thermodynamic stability is typically expressed in term of an overall formation or stability constant i.e , K f ΔG o is thermodynamic Property Hence formation Constant is measure as thermodynamic stability. Higher – Ve value of Δ G higher indicate that the complex formation is more stable and spontaneous in nature. The magnitude of formation constant is measure the stability of a complex. Stepwise Formation Constant When more than one coordinated water molecule of [M(H 2 O)] x + is replaced by the ligand L, several steps of reaction considered,

4. Overall Formation Constant( β n ) K1 , K2 & Kn are called stepwise formation const or stability constant, ßn is overall formation constant.

5. Increase the steric hindrance as the no of ligands involved, the entropy is less and according to the eq , Δ G= Δ H – T Δ S Δ S G is more – ve and K 1 > K 2 and so on K inetic Stability The kinetic stability depends on the speed of the reaction, which depend on activation energy (Ea) and rate of the reaction. Kinetically Inert Kinetically Labile Inert complex gives substitution reaction with difficulties t 1/2 > 1 min Labile complexes gives substitution reaction rapidly t 1/2 < 1min

6. Kinetic Vs. Thermodynamic Stability There is no relation between the thermodynamic stability/ instability and kinetic lability /inertness toward substitution reaction. Highly Inert

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