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• We can see that all six water ligands have been replaced by four chloride ions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

• 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??is negative
• The driving force behind the reaction is entropy
• The Gibbs equation reminds us of the link between enthalpy and entropy:

ΔG??= ΔHreaction??– TΔSsystem?

• 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)?+ EDTA4-?(aq) → [CoEDTA]2-?(aq) + 6H2O?(l)

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