Ligand substitution in complexes 214-215

Question 1

Define ligand substitution

Answer 1

Ligand substitution is a reaction in which one ligand in a complex ion is replaced by another ligand.

Question 2

Why is the shape of [Cu(NH₃)₄(H₂O)₂]²⁺ strictly described as a distorted octahedral shape?

Answer 2

• The copper-oxygen bonds are longer than the copper-nitrogen bonds, so the shape is strictly described as a distorted octahedral shape.

Question 3

Write out the reaction that occurs when an aqueous solution of [Cu(H₂O)₆]²⁺ reacts with an excess of ammonia solution.

Answer 3

[Cu(H₂O)₆]²⁺(aq) + 4NH₃(aq) ⇔ [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 4H₂O(l)

• In the reaction, four of the water ligands are replaced by four ammonia ligands. Ligand substitution has taken place because one type of ligand has been replaced by another ligand.

Question 4

If you add ammonia to a solution of [Cu(H₂O)₆]²⁺ complex ions what would be observed?

Answer 4

The initial solution of [Cu(H₂O)₆]²⁺ ions has a characteristic pale blue colour .On addition of a small amount of ammonia, a pale blue precipitate of copper(II) hydroxide, Cu(OH)_2,forms. On addition of an excess of aqueous ammonia, the pale blue precipitate dissolves and a deep blue solution is formed containing [Cu(NH₃)₄(H₂O)₂]²⁺ ions.

Question 5

Explain the colour changes when concentrated hydrochloric acid is added to an aqueous solution containing copper(II) ions. And write the equilibrium that occurs.

Answer 5

The pale blue solution initially forms a green solution before finally turning yellow. This reaction exists in equilibrium and can be reversed by adding water to the yellow solution to return it to the original blue colour.

[Cu(H₂O)₆]²⁺(aq) + 4Cl^-(aq) ⇔ [CuCl₄]^2-(aq) + 6H₂O(l)

Question 6

Explain why [Cu(H₂O)₆]²⁺complex ion has six ligand, but the [CuCl₄]^2- complex ion has only four ligands.

Answer 6

This occurs because the chloride ligands are larger than the water ligands and have stronger repulsions, so fewer chloride ligands can fit around the central metal ion. [CuCl₄]^2- has a tretrahedral shape, as in a CH₄ molecule.

Question 7

State the colour change and equilibrium reaction when a solution of [Co(H₂O)₆]²⁺ complex ions has concentrated hydrochloric acid added to it.

Answer 7

An aqueous solution of cobalt(II) ions contains [Co(H₂O)₆]²⁺ complex ions, which have a characteristic pale pink colour. If concentrated hydrochloric acid is then added, the solution forms a dark bue solution. The concentrated HCl provides a high concentration of chloride ions. The six water molecules in the complex ion are replaced by four chloride ions.

[Co(H₂O)₆]²⁺(aq) + 4Cl^-(aq) ⇔ [CoCl₄]^2-(aq) + 6H₂O(l)