Ligand Substitution

Write the equation for the reaction of Hexaaquacopper(II) with concentrated Hydrochloric Acid (Cl^-). State the colour change.

• Equation:[Cu(H_2O)_6]^{2+}+4Cl^{-}\rightarrow[CuCl_4]^{2-}+6H_2O

• Colour: Pale Blue solution → Yellow/Green solution.

Why does the coordination number change from 6 to 4 when water ligands are replaced by chloride ligands?

• Chloride ions (Cl⁻) are larger than water molecules. There is not enough space for 6 chloride ions to fit around the central metal ion (steric hindrance/repulsion).

State the shape of the complex ion [CoCl₄]^2- and its colour.

• Shape: Tetrahedral.

• Colour: Blue.

  (Remember: Cobalt goes from Pink water → Blue chloride).

Define the Chelate Effect.

• The substitution of monodentate ligands by bidentate or multidentate ligands leads to a more stable complex. This is driven by a large increase in entropy.

Explain, in terms of entropy (∆S), why the reaction below is feasible:

[Cu(H_2O)_6]^{2+} + EDTA^{4-} \rightarrow [Cu(EDTA)]^{2-} + 6H_2O

• There are 2 moles of particles on the left and 7 moles of particles on the right.

• This represents a large increase in disorder (positive ∆S).

• Therefore, ∆G becomes negative (feasible).

Haemoglobin transports oxygen. Why is Carbon Monoxide (CO) toxic in terms of ligand substitution?

• CO is a better ligand than O₂.

• It binds irreversibly (strong coordinate bond) to the Fe²⁺ ion in haemoglobin, replacing the oxygen.

• This prevents the haemoglobin from carrying oxygen to the body's tissues.

State the observation when excess Ammonia is added to a solution of Hexaaquacopper(II).

• [Cu(H_{2}O)_{6}]^{2+}{}_{(aq)}+4NH_{3}{}_{(aq)}\rightarrow [Cu(NH_{3})_{4}(H_{2}O)_{2}]^{2+}{}_{(aq)}+4H_{2}O{}_{(l)}

• Pale blue solution forms a pale blue precipitate, which redissolves to form a deep blue solution.

State the observation when Concentrated HCl is added to a solution of Hexaaquacobalt(II).

[Co(H_{2}O)_{6}]^{2+}\text{\ (pink)}+4Cl^{-}\rightleftharpoons [CoCl_{4}]^{2-}\text{\ (blue)}+6H_{2}O