Question

What is meant by stability of coordination compounds in solution? State the factors which govern the stability of complexes.

Answer 1

Formation of complex in solution takes place by the stepwise addition of the ligands to the metals ion. If there are n steps then there should be in equilibrium, here n represents the coordination number of the metal ion. And every equilibrium will have its equilibrium constant.

Complete step by step answer:
The equilibrium constants for each step of addition of ligands are known as stepwise formation constants or stepwise stability constants.
Suppose we put metal ion M and some monodentate ligand L together in solution and we assume that no insoluble product and any species containing more than one metal ion are formed, then the formation of the complex MLn may be supposed to take place in the following manner.

$$M + L\underset{{}}{\overset{{{K_1}}}{\longleftrightarrow}}ML,{K_1} = \dfrac{{[ML]}}{{[M][L]}} \\\ ML + L\overset {{K_2}} \leftrightarrows M{L_2},{K_2} = \dfrac{{[M{L_2}]}}{{[ML][L]}} \\\ M{L_{n - 1}} + L\overset {{K_n}} \leftrightarrows M{L_n},{K_n} = \dfrac{{[M{L_n}]}}{{[M{L_{n - 1}}][L]}} \\\$$

With few exceptions, the values of the successive stability constants decrease regularly from K1 to Kn. The overall formation constant or stability constant will be given as-
$\beta = {K_1} \times {K_2} \times ......{K_{n - 1}} \times {K_n}$
The higher the value of stability constant for a complex ion, the greater will be its stability of that complex.
Some factors that affects the stability of metal complex are-

1. Properties of metal ion - for a given ligand, the stability constant of the complex of metallic ions having the same charge on them decreases with the increase in the size of metal ion.
   For a given ligand, the stability of complex ions having metallic ions of almost similar size but different charges on them increases with the increase in the charge of the central metal ion.
   For the central metal ion with high polarizing power gives more stable complexes.
2. Properties of the ligand- if the ligand is smaller, it can approach the metal closely and form stable bonds. A highly charged ligand will also form a strong bond with metal.
3. For neutral ligands, the larger the magnitude of dipole moment, the greater is the stability of complexes.
   The more basic a ligand is, the more easily it will donate the electron pair and form a stable complex.
   The ligands which are capable of forming pi- bonds with metals give a more stable complex.

Note: The stability of complexes also depends on the stability of chelates as chelates having 6- membered rings including the metal are more stable than those having 5- membered, which will be more stable than 4- membered rings. Greater the number of chelate rings, greater will be stability. Resonance enhances the stability of complexes. Chelated complexes are known to be more stable than non- chelated complexes.