3.2.5.2: Substitution Reactions

• Monodentate ligands:

  Molecule or ion that forms only 1 coordinate bond to central metal ion/atom in complex by donating (1) pair of electrons
• Bidentate ligands:

  Molecule or ion that can form two coordinate bonds with central metal ion/atom by donating 2 pairs of electrons
• Multidentate ligands:

  Molecule or ion that can form multiple (at least more than 2) coordinate bonds with central metal ion/atom by donating multiple pairs of electrons
• Give examples of monodentate ligands:

  • H2O
  • NH3
  • Cl-
  • CN-
• Give examples of bidentate ligands:

  • C2O4 2- (oxalate/ethandioate ion)
  • H2NCH2CH2NH2 (1,2-diaminoethane)
• Give an example of a multidentate ligand:

  • EDTA 4- (ethylenediaminetetraacetic acid)
• Define: Chelation

  Substitution of (monodentate) ligands by bidentate or multidentate ligands to form more stable complex
• Chelation results in the formation of more stable complexes ∵:

  • Increase in no. of particles/molecules from reactants to products
  • Increase in disorder -> increase in ∆S/greater ∆S
  • Where ∆H is near zero/negligible ∵ same no. of metal ion-ligand coordinate bonds broken as they are formed (at a given temperature)
  • …Which means that ∆G is negative ∴ reaction is feasible (∵ stable complex formed)
• Complete substitution within complexes occurs:

  • without any change in coordination number of the complex
• Incomplete substitution within complexes occurs:

  • With a change in coordination number & shape in the complex
• If new complex is more stable than previous complex, reaction is:

  • less likely to be in equilbrium/reversible
• If new ligand forms stronger bonds than old ligand in complex, reaction is:

  • less likely to be in equilibrium/reversible