Ncert Exercises & Solutions

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

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 + 3 L \leftrightarrow M L_{3}

S t a b i l i t y c o n s \tan t, β = \frac{[M L_{3}]}{[M] {[L]}^{3}}

For this reaction, the greater the value of the stability constant, the greater is the proportion of

M L_{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.

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

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

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