Stability of complexesThe substitution of monodentate ligand with a bidentate or a multidentate ligand leads to a more stable complex. This is called the chelate effect [Cu(H2O)6 ]2+ (aq) + EDTA4- (aq) [Cu (EDTA)]2- (aq) + 6H2O (l) The copper complex ion has changed from having unidentate ligands to a multidentate ligand. In this reaction there is an increase in the entropy because there are more moles of products than reactants (from 2 to 7), creating more disorder. This chelate effect can be explained in terms of a positive entropy change in these reactions as more molecules of products than reactants Free energy ΔG will be negative as ΔS is positive and ΔH is small. The enthalpy change is small as there are similar numbers of bonds in both complexes. The stability of the EDTA complexes has many applications. It can be added to rivers to remove poisonous heavy metal ions as the EDTA complexes are not toxic. It is in many shampoos to remove calcium ions present in hard water, so helping lathering. [Co(NH3 )6 ]2+ + 3NH2CH2CH2NH2 [Co(NH2CH2CH2NH2 )3 ]2+ + 6NH3 This reaction has an increase in entropy because of the increase in moles from 4 to 7 in the reaction. ΔS is positive. Its enthalpy change ΔH is close to zero as the number of dative covalent and type (N to metal coordinate bond) are the same so the energy required to break and make bonds will be the same. Therefore Free energy ΔG will be negative and the complex formed is
126.96.36.199 Substitution reactions (A-level only)
Bidentate and multidentate ligands replace monodentate ligands from complexes. This is called the chelate effect.
Students should be able to explain the chelate effect, in terms of the balance between the entropy and enthalpy change in these reactions.