Name: Nomenclature of Coordination Compounds - I
Uploaded: 2019-07-19
Duration: 7 min 4 s
Description: All the chemical compounds are assigned a name, which is done according to a specific rule. How are coordination compounds named? Let’s watch the video to fi...

Question

Select a complex formation reaction and write an expression for the stability constant of the complex species. What information is conveyed regarding the strength of ligands from the stability constant values of their complexes with a metal ion? Illustrate your answer with examples of monodentate ligands.

Accepted Answer

Consider the following reaction of formation of complex:

${Cu}^{2 +} + {NH}_{3} ⇌ {[Cu ({NH}_{3})]}^{2 +}, K_{1} = \frac{{[Cu ({NH}_{3})]}^{2 +}}{[{Cu}^{2 +} (aq)] [{NH}_{3} (aq)]}$

$Cu {({NH}_{3})}^{2 +} + {NH}_{3} \to Cu {({NH}_{3})}_{2}^{2 +}$ ^, $k_{2} = \frac{[Cu {({NH}_{3})_{2}}^{2 +}]}{[\begin{matrix} Cu ({NH}_{3})^{2 +}] & [{NH}_{3} \end{matrix}]}$

${[Cu ({NH}_{3})]}_{2}^{2 +} + {NH}_{3} ⇌ {[Cu ({NH}_{3})]}_{3}^{2 +}, K_{3} = \frac{{[Cu ({NH}_{3})]}_{3}^{2 +}}{{[Cu ({NH}_{3})]}_{2}^{2 +} [{NH}_{3} (aq)]} {[Cu ({NH}_{3})]}_{3}^{2 +} + {NH}_{3} ⇌ {[Cu ({NH}_{3})]}_{4}^{2 +}, K_{3} = \frac{{[Cu ({NH}_{3})]}_{4}^{2 +}}{{[Cu ({NH}_{3})]}_{3}^{2 +} [{NH}_{3} (aq)]}$

Hence, overall stability, $β$ = $\frac{[Cu {({NH}_{3})_{4}}^{2 +}]}{\begin{matrix} [{Cu}^{2 +}] & {[({NH}_{3})]}^{4} \end{matrix}}$

The stability constant or dissociation constant is defined as reciprocal of formation constant.

The less the value of $K$ , the more stable is a ligand.

From the data on stability constants, the following generalisations are made:

(i) For a given metal and ligand, the stability increases with an increase in charge on the metal ion. Thus, complex compounds having charge 3+ on the metal ion are more stable than the corresponding compounds in 2+ oxidation state. However, for the divalent ions of the first transition elements, irrespective of the ligand involved,

The stabilities vary in the order: ${Mn}^{2 +} < {Fe}^{2 +} < {CO}_{2} < {Ni}^{2 +} < {Cu}^{2 +} < {Zn}^{2 +}$ (Irving-Williams order)

(ii) 'Class a' acceptors such as metal ions of group I and 2, the inner-transition elements and the early members of the transition series (group 3 to 6) form most stable coordination compounds with ligands containing $N, O or F$ donor atoms.

(iii) 'Class b' acceptors like the metal ions of $Rh, Pd, Ag, Jr, Au and Hg$ having nearly full orbitals form stable complexes with the ligands containing heavier members of $N, O or F$ groups as donor atoms.

(iv) The complexes formed by chelating ligands are more stable than their monodentate analogues due to reduced strain in the chelate. The effect is called the chelating effect. A chelated complex or cyclic complex is usually more stable than a non-chelated complex or an open one. For example, ${[Cu {(en)}_{2}]}^{2 +}$ is more stable than a ${[Cu {({NH}_{3})}_{4}]}^{2 +}$ ion.
The chelate effect is maximum for the 5- and 6-membered rings. Chelation is of great importance in analytical chemistry and biological systems.

(v) A cyclic multidentate further increases the stability provided there are no unfavourable steric effects. This is termed a macrocyclic effect.

Important Questions on Coordination Compounds

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