Question

When an excess of aqueous $KCN$ solution added to an aqueous solution of copper sulphate, the complex ${{[Cu{{(CN)}_{4}}]}^{2-}}$ formed. On passing ${{H}_{2}}S$ gas through this solution no precipitate of $CuS$ is formed because:
(A) sulphide ions cannot replace $C{{N}^{-}}$ ions
(B) ${{[Cu{{(CN)}_{4}}]}^{2-}}$does not give $C{{u}^{+2}}$ ions
(C) sulphide ions from ${{H}_{2}}S$do not form complexes
(D) sulphide ions cannot replace sulphate ions from copper sulphate solution

Answer 1

Hint: In case of polydentate ligands, coordination complexes stability depends upon chelate rings. This is known as the chelation effect. The stability of complexion in the solution depends on the degree of dissociation between two species in the state of equilibrium.

Complete step by step solution:
The formation of a complex compound in several steps where each step is a reversible and stepwise equilibrium constant is known as stepwise formation constant.
The factors on which stability of complex compound depends:
(1) charge on the central metal ion- small size and high charge on metal ion increase the stability of complex compounds.
(2) nature of metal ion – stability order is 3d>4d>5d
(3) nature of ligands – strong field ligands form stable complexes, for example, $C{{N}^{-}}, CO$, etc. which are strong field ligands form stable complexes.

The dissociation constant of compounds is defined as the reciprocal of the formation or stability constant.
Given that, aqueous KCN solution added to an aqueous solution of copper sulphate the reaction as follows,
$CuS{{o}_{4}}(aq)+4KCN\to {{K}_{2}}[Cu{{(CN)}_{4}}]+{{K}_{2}}S{{O}_{2}}(aq)$
When added excess of aqueous solution KCN,
\[{{[Cu{{({{H}_{2}}O)}_{4}}]}^{2+}}+4C{{N}^{-}}\to {{[Cu{{(CN)}_{4}}]}^{2-}}+{{H}_{2}}O\]
Due to the strong field ligand $C{{N}^{-}}$, it forms a stable metal complex with copper ${{[Cu{{(CN)}_{4}}]}^{2-}}$, which does not give $C{{u}^{+2}}$ ions and hence On passing ${{H}_{2}}S$ gas through this solution no precipitate of CuS is formed.

So, the correct answer is option B.

Note: Neutral molecules or anions bond to a central metal atom or ion in Lewis acid-base reaction, the product formed in this reaction is coordination complex. The coordination covalent bond means the bond involved in the formation of the coordination complex.