191.

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

The stability of a complex in solution refers to the degree of association between the two species involved in the state of equilibrium. The magnitude of the (stability or formation) equilibrium constant for the association, quantitatively express the stability.
The complex formation is a Lewis acid-base concept.
                             K = M

     ${\mathrm{M}}^{\mathrm{a}} + {\mathrm{nLX}}^{-} \rightleftharpoons {\left[\mathrm{MLn}\right]}^{\mathrm{b}+}$

Here a⁺, x^– and b⁺ are the charges on metal, ligand and complex respectively. Charge balance requires that
(a⁺)+ n(x^–) = (b⁺)

The stability constant; K of the complex can be given as   

$\mathrm{K} = \frac{\left[(\mathrm{MLn}{)}^{\mathrm{b}+}\right]}{\left[{\mathrm{M}}^{\mathrm{n}+}\right] {\left[{\mathrm{L}}^{\mathrm{x}-}\right]}^{\mathrm{n}}}$

Higher the value of K, more the stability of the coordination compound.

The stability of the coordination compound depends on:
(i) Nature of the ligand: Chelating ligands form strong and more stable complexes than the monodendate ligands.
The $\mathrm{\pi }$ - bond ligands form more stable complexes than the σ bonded complex.

(ii) Nature of the metal atom/ion: Small, highly charged metal ions form more stable complexes than large size, lowly charged metal ion.