Exchanging Ligands
- Ligand exchange (or ligand substitution) is when one ligand in a complex is replaced by another
- Ligand exchange forms a new complex that is more stable than the original one
- The ligands in the original complex can be partially or entirely substituted by others
- The complex ion can change its charge or remain the same depending on the ligand involved
- There are no changes in coordination number, or the geometry of the complex, if the ligands are of a similar size
- But, if the ligands are of a different size, for example water ligands and chloride ligands, then a change in coordination number and the geometry of the complex will occur
- Addition of a high concentration of chloride ions (from conc HCl or saturated NaCl) to an aqueous ion leads to a ligand substitution reaction.
- The Cl- ligand is larger than the uncharged H2O and NH3 ligands so therefore ligand exchange can involve a change of co-ordination number
- For example when concentrated hydrochloric acid is added slowly and continuously to a copper(II) sulfate solution the colour changes from blue to green then finally yellow
- The equation for this reaction is
[Cu(H2O)6]2+ (aq) + 4Cl- (aq) ⇌ [CuCl4]2- (aq) + 6H2O (l)
- We can see that all six water ligands have been replaced by four chloride ions
- This reaction involves a change in coordination number from 6 to 4
- Note that despite the charge on the complex changing from +2 to -2, there has been no change in oxidation number of the copper
- We can also see that this reaction is reversible, which helps to explain the observed colour change
- The hexaaquacopper(II) ion is blue
- The tetrachlorocuprate(II) ion is yellow
- The green colour is due to a mixture of the blue and yellow complex ions
- A similar reaction also takes place with cobalt resulting in a blue solution and a change in coordination number from 6 to 4
[Co(H2O)6]2+ (aq) + 4Cl- (aq) ⇌ [CoCl4]2- (aq) + 6H2O (l)
Examiner Tip
Be careful: If solid copper chloride (or any other metal) is dissolved in water it forms the aqueous [Cu(H2O)6]2+ complex and not the chloride [CuCl4 ]2- complex