Edexcel A Level Chemistry:复习笔记6.3.4 Ligand Exchange

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  • We can see that all six water ligands have been replaced by four chloride ions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

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

The Chelate Effect & 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)

Chelation-with-EDTA

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

 

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