10. STABILITY OF COORDINATION COMPOUNDS

Stability of coordination compounds :

The stability of a coordination compound [ML_n] is measured in terms of the stability constant (equilibrium constant) given by the expression,

bn = [ML_n]/[M(H₂O)_n][L]ⁿ

for the overall reaction :

M(H₂O)_n + nL  ML_n + nH₂O

By convention, the water displaced is ignored, as its concentration remains essentially constant. The above overall reaction takes place in steps, with a stability (formation) constant, K₁, K₂, K₃, ...... K_n for each step as represented below :

M(H₂O)_n + L  ML(H₂O)_n–1 + H₂O

K₁ = [ML(H₂O)_n–1] / {[M(H₂O)_n][L]}

ML_n–1 (H₂O) + L  ML_n + H₂O

K_n = [ML_n] / {[ML_n–1 (H₂O)] [L]}

M(H₂O)_n + nL ML_n + nH₂O

bn = K₁ x K₂ x K₃ x ........ x K_n

bn, the stability constant, is related to thermodynamic stability when the system has reached equilibrium. Most of the measurements have been made from aqueous solutions, which implies that the complex is formed by the ligand displacing water from the aqua complex of the metal ion. Ignoring the charge and taking L as an unidentate ligand, the stepwise formation of the complex is represented as shown above. K₁, K₂, K₃ ..... K_n representing the stepwise stability (or formation) constants.

The above is thermodynamic stability criteria, there can be another kind of stability called Kinetic Stability, which measures the rate of ligand replacement.

Some important generalisation regarding stability constants :

For a given metal and ligand the stability is generally greater when the charge on the metal ion is greater. Thus, stability of coordination entities of ions of charge 3+ is greater than the entities of 2+ ions. Further, for the divalent ions of the first row transition elements, irrespective of the ligand involved, the stabilities vary in the Irving-Williams order :

Mn^II < Fe^II < Co^II < Ni^II < Cu^II > Zn^II

This order is according to the size of the ions, smaller the size of the ion or greater the charge density on the metal greater is the stability of the complex.

In F¯, Cl¯, Br¯, I¯ ; F¯ forms strongest complexes due to small size & hence high charge density.

The stability also depends on the formation of chelate rings. If L is an unidentate ligand and L–L, a didentate ligand and if the donor atoms of L and L–L are the same element, then L–L will replace L. The stabilisation due to chelation is called the chelate effect. It is of great importance in biological systems and analytical chemistry. The chelate effect is maximum for the 5- and 6-membered rings. In general, rings provide greater stability to the complex.