Aqueous solution of copper sulphate contains Cu2+ ions in the form of complex entity, [Cu(H2O)4]2+ and H2O ligand is a weak ligand. When excess of KCN is added, a new coordination entity, [Cu(CN)4]2- is formed due to the following reaction :
Cyanide ligand, CN- is a strong field lignad and stability constant of [Cu(CN4)]2- is quite large and thus practically no Cu2+ ions are left in solution. On passing H2S gas, no CuS is formed due to non- availability of Cu2+ ions in solution.