6.3.4 Ligand Exchange

• 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• We can see that all six water ligands have been replaced by four chloride ions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) 

Exam Tip

• The replacement of monodentate ligands with bidentate and multidentate ligands in complex ions is called the chelate effect
• It is an energetically favourable reaction, meaning that ΔG^ꝋ is negative
• The driving force behind the reaction is entropy
• The Gibbs equation reminds us of the link between enthalpy and entropy:

ΔG^ꝋ = ΔH_reaction^ꝋ – TΔS_system^ꝋ

• Reactions in solution between aqueous ions usually come with relatively small enthalpy changes
• However, the entropy changes are always positive in chelation because the reactions produce a net increase in the number of particles
• A small enthalpy change and relative large positive entropy change generally ensures that the overall free energy change is negative
• For example, when EDTA chelates with aqueous cobalt(II) two reactants becomes seven product species

[Co(H₂O)₆ ]²⁺ (aq) + EDTA^4- (aq) → [CoEDTA]^2- (aq) + 6H₂O (l) 

The ligand EDTA readily chelates with aqueous transition metal ions in an energetically favourable reaction