Ligand Exchange (Edexcel International A Level Chemistry): Revision Note
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We can see that all six water ligands have been replaced by four chloride ions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 Tips and Tricks
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
Entropy & Stability
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 ΔGꝋ is negative
The driving force behind the reaction is entropy
The Gibbs equation reminds us of the link between enthalpy and entropy:
ΔGꝋ = ΔHreactionꝋ – TΔSsystemꝋ
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
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