Formation of a complex in solution is an equilibrium reaction. It may be represented as \(\mathrm{M}+4 \mathrm{~L} \rightleftharpoons \mathrm{ML}_4\) The equilibrium constant of this reaction is the measure of stability of the complex. Hence the equilibrium constant is
also called as stability constant or Instability constant may be defined as equilibrium constant for reverse reaction. The formation of above complex may also be written in successive steps:
\(\begin{array}{ll}\mathrm{M}+\mathrm{L} \rightleftharpoons \mathrm{ML} & \mathrm{K}_1 \\ \mathrm{ML}+\mathrm{L} \rightleftharpoons \mathrm{ML}_2 & \mathrm{~K}_2 \\ \mathrm{ML}_2+\mathrm{L} \rightleftharpoons \mathrm{ML}_3 & \mathrm{~K}_3 \\ \mathrm{ML}_3+\mathrm{L} \rightleftharpoons \mathrm{ML}_4 & \mathrm{~K}_4\end{array}\)
Stability constant is written as
\(\beta_4=\mathrm{K}_1 \mathrm{~K}_2 \mathrm{~K}_3 \mathrm{~K}_4 \text {. }\)
Greater the stability constant, stronger is the metal-ligand bond.
The stability of complex will depend on
(a) nature of metal
(b) Oxidation state of metal
(c) Nature of ligand e.g. chelating ligand form stabler complexes
(d) Greater the basic strength of the ligand, more will be the stability.