3.2.5.2 Substitution reactions

• H2O, NH3 and Cl− can act as monodentate ligands.
• The ligands NH3 and H2O are similar in size and are uncharged.
• Exchange of the ligands NH3 and H2O occurs without change of co-ordination number (eg Co2+ and Cu2+).
• Substitution may be incomplete (eg the formation of [Cu(NH3)4(H2O)2] 2+).
• The Cl− ligand is larger than the uncharged ligands NH3 and H2O
• Exchange of the ligand H2O by Cl– can involve a change of co-ordination number (eg Co2+, Cu2+ and Fe3+).
• Ligands can be bidentate (eg H2NCH2CH2NH2 and C2O4 2–).
• Ligands can be multidentate (eg EDTA4–).
• Haem is an iron(II) complex with a multidentate ligand.
• • Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood.
  • Carbon monoxide is toxic because it replaces oxygen coordinately bonded to Fe(II) in haemoglobin.
• • Bidentate and multidentate ligands replace monodentate ligands from complexes.
  • This is called the chelate effect.
• 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.
• This chelate effect can be explained in terms of a positive entropy change in these reactions as there are more molecules of products than reactants.

  • 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.
• • The enthalpy change is small as there are similar numbers of bonds in both complexes.
  • Free energy ΔG will be negative as ΔS is positive and ΔH is small.
• Students could carry out test-tube reactions of solutions of metal aqua ions with ammonia or concentrated hydrochloric acid.

  CONCENTRATED HYDROCHLORIC ACID WITH COPPER:

  • Yellow-green solution formed
  • [Cu(H2O)6]2+ + 4Cl- -> [Cu(Cl)4 ]2- + 6H2O
  • Tedrahedral

  CONCENTRATED HYDROCHLORIC ACID WITH COBALT

  • Blue solution formed
  • [Co(H2O)6 ]2+ + 4Cl -> [CoCl4 ]2- + 6H2O
• Explain the chelate effect, in terms of the balance between the entropy and enthalpy change in these reactions.

  Suggest why the value of enthalpy change of the reaction is close to zero

  • the number of dative covalent and TYPE of bond(SPECIFY depending on context of question) are the same
  • the energy required to break and make bonds will be the same