#### Question

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

#### Solution 1

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 ↔ ML_{3}

Stability constant, beta = `[ML_3]/([M][L]^3)`

For this reaction, the greater the value of the stability constant, the greater is the proportion of ML_{3} 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.

**Basic nature of the ligand: **A more basic ligand will form a more stable complex.

**Presence of chelate rings: **Chelation increases the stability of complexes.

#### Solution 2

Formation of a complex in solution is an equilibrium reaction. It may be represented as M + 4L ↔ ML4

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:

M + L ⇌ ML K_{1}

ML + L ⇌ ML_{2} K_{2}

ML_{2} + L ⇌ ML_{3} K_{3}

ML_{3} + L ⇌ ML_{4} K_{4}

Stability constant is written as

β_{4}=K_{1}K_{2}K_{3}K_{4}.

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.