The stability of a complex in a solution refers to the degree of association between the two species involved in a state of equilibrium. Stability can be expressed quantitatively in terms of stability constant or formation constant.

M +  3L  ↔   ML₃

Stability Constant, β  =  [ML₃] / [M][L³]

For this reaction, the greater the value of the stability constant, the greater is the proportion of ML₃ in the solution.

Stability can be of two types:

(a) Thermodynamic stability:

The extent to which the complex will be formed or will be transformed into another species at the point of equilibrium is determined by thermodynamic stability.

(b) Kinetic stability:

This helps in determining the speed with which the transformation will occur to attain the state of equilibrium.

Factors that affect the stability of a complex are:

(a) Charge on the central metal ion: Thegreater the charge on the central metal ion, the greater is the stability of the complex.

(b) Basic nature of the ligand: A more basic ligand will form a more stable complex.

(c) Presence of chelate rings: Chelation increases the stability of complexes.