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Ligand Substitutions & Kstab

Chemistry: Cambridge International A Level, topic 2.4.9

Ligand Subsitution & Kstab

Complex metal ions have ligands which can be substituted. Kstab is defined as the equilibrium constant for the formation of the complex ion in a solvent from its constituent ions or molecules.

The reaction

The reaction

  • The complex ion [ Cu(H2O)6 ]2+ undergoes a ligand substitution reaction where H2O is replaced by Cl-.
  • The reaction is reversible.
  • [ Cu(H2O)6 ]2+ + 4Cl−  ⇋ \:\leftrightharpoons\: [ CuCl4 ]2- + 6H2O
  • The equilibrium can be pushed to the right using concentrated hydrochloric acid (HCl).
  • The reaction can be reversed by adding water. This will push the equilibrium to the left.
[ Cu(H2O)6 ]2+ vs. [ CuCl4 ]2-

[ Cu(H2O)6 ]2+ vs. [ CuCl4 ]2-

  • In the reaction, the copper goes from having six water ligands to having four chloride ligands.
  • The co-ordination number changes from six to four in the reaction.
  • This is because the chloride ions are larger than the water molecules.
  • There is only space for four chloride ions around the copper.
Reversible reaction

Reversible reaction

  • As this reaction is reversible, we can form an equilibrium expression. This is known as Kstab, or the stability constant.
  • For the below reaction:
  • [ Cu(H2O)6 ]2+ + 4Cl−  ⇋ \:\leftrightharpoons\: [ CuCl4 ]2- + 6H2O
  • Kstab = [CuCl4][Cu(H2O)6][Cl−]4\Large\frac {[CuCl_4]}{[Cu(H_2O)_6][Cl^-]^4}
  • Looking at the expression, we can see a large Kstab results in the formation of a stable complex ion.
Colour change

Colour change

  • The colour change observed should be from blue to yellow.
  • However, there is normally a mixture of [ Cu(H2O)6 ]2+ and [ CuCl4 ]2- ions.
  • This means the observed colour change is from blue to green (blue + yellow = green).

Substituting Water for Chlorine in a Complex Cobalt Ion

Let's go through another example. The chemistry in this reaction with a complex cobalt ion is exactly the same as the reaction with the with a complex copper ion.

The reaction

The reaction

  • The complex ion [ Co(H2O)6 ]2+ undergoes a ligand substitution reaction where H2O is replaced by Cl-.
  • The reaction is reversible.
  • [ Co(H2O)6 ]2+ + 4Cl−  ⇋ \:\leftrightharpoons\: [ CoCl4 ]2- + 6H2O
  • The equilibrium can be pushed to the right using concentrated hydrochloric acid (HCl).
  • The reaction can be reversed by adding water. This will push the equilibrium to the left.
[ Co(H2O)6 ]2+ vs. [ CoCl4 ]2-

[ Co(H2O)6 ]2+ vs. [ CoCl4 ]2-

  • In the reaction, the cobalt goes from having six water ligands it to having four chloride ligands.
  • The co-ordination number changes from six to four in the reaction.
  • This is because the chloride ions are larger than the water molecules.
  • There is only space for four chloride ions around the cobalt.
Reversible reaction

Reversible reaction

  • As this reaction is reversible, we can form an equilibrium expression. This is known as Kstab, or the stability constant.
  • For the below reaction:
  • [ Co(H2O)6 ]2+ + 4Cl−  ⇋ \:\leftrightharpoons\: [ CoCl4 ]2- + 6H2O
  • Kstab = [CoCl4][Co(H2O)6][Cl−]4\Large\frac {[CoCl_4]}{[Co(H_2O)_6][Cl^-]^4}
  • Looking at the expression, we can see a large Kstab results in the formation of a stable complex ion.
Colour change

Colour change

  • The colour change observed should be from pink [ Co(H2O)6 ]2+ ions to blue [ CoCl4 ]2- ions.

Test yourself on Ligand Substitutions & Kstab

  • What colour change is observed when [ Cu(H2O)6 ]2+ reacts with chloride ions to form [ CuCl4 ]2-?

  • When [ Cu(H2O)6 ]2+ reacts to form [ CuCl4 ]2-, what happens to the co-ordination number of the complex ion?

  • Which of the follwoing are true about the formation of [CuCl4]2- from [Cu(H2O)6]2+?

  • In the reaction [ Cu(H2O)6 ]2+ + 4Cl−  ⇋ \:\leftrightharpoons\: [ CuCl4 ]2- + 6H2O, how do you push the equilibrium to the right?

  • What colour change is observed when [ Co(H2O)6 ]2+ reacts with chloride ions to form [ CoCl4 ]2-?

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