Colour, Magnetism And Stability Of Co-ordination Compounds

Colour, Magnetism And Stability Of Co-ordination Compounds

(a) Colour

(i) Colour of a complex depends on the metal, its oxidation state, and its ligands.

(ii) Pale blue ${\left[Cu{\left(H_{2}O\right)}_4\right]}^{2+}$ can be converted into dark blue ${\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}$ by adding NH_3(aq).

(iii) A partially filled set of d orbitals is usually required for a complex to be coloured.

(iv) So, d⁰ metal ions are usually colourless. Exceptions: MnO₄^- and$Cr{O}_4^{2-}$.

(v) Coloured compounds absorb visible light.

(vi) The colour perceived is the sun of the light not absorbed by the complex.

(vii) The amount of absorbed light wavelength is an absorption spectrum for a complex.

(viii) For example, the absorption spectrum for ${\left[Ti{\left(H_{2}O\right)}_6\right]}^{3+}$ has a maximum absorption occurs at 510 nm (green and yellow).

(b) Magnetism

(i) Many transition metal complexes are paramagnetic (i.e. they have unpaired electrons).

(ii) ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$has no unpaired electrons, but ${\left[Co{F}_6\right]}^{3-}$ has four unpaired electrons per ion.

Illustration : Which of the two compounds is more stable and why?

[K₄[Fe(CN)₆], K₃[Fe(CN)₆]

Solution: K₃[Fe(CN)₆] is more stable because in this compound Fe is in oxidation state III while in the first compound Fe is in oxidation state II which can be oxidized to oxidation state III.

STABILITY OF COORDINATION COMPOUNDS IN SOLUTION

The stability of a complex is defined as a measure of resistance to the replacement of a ligand by some other ligand. As the dissociation involves an equilibrium between the dissociated components (ions or molecules) and the undissociated complex, the stability of a complex in solution can be expressed in terms of equilibrium constant of the dissociation equilibrium. This constant is called the instability constant. For example, for the complex [Cu(NH₃)₄]²⁺ in solution, we have

${\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}\rightleftharpoons C{u}^{2-}+4N{H}_3$

Instability constant, $K_i=\frac{\left[C{u}^{2+}\right]{\left[N{H}_3\right]}^4}{{\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}}$

If instead of dissociation, we talk of the formation of a complex ion in the solution. The reaction will be reverse of the above reaction, i.e., we have

$C{u}^{2+}+4N{H}_3\rightleftharpoons {\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}$

The equilibrium constant for this reaction is, therefore, called the stability constant.

Representing it by K, we have

Stability constant, $K=\frac{{\left[Cu{\left(N{H}_3\right)}_4\right]}^{2+}}{\left[C{u}^{2+}\right]{\left[N{H}_3\right]}^4}$

Thus, stability constant is the reciprocal of the instability constant (K = 1/K_i).

K is known as the stability constant of the complex. The numerical value of the stability constant is a measure of stability of the complex in solution. Greater the magnitude of the stability constant, more stable is the complex.