Substitution reactions

Ligand substitution

Where one ligand is replaced by another ligand for more stability

NH3 and H20

• These ligands are similiar in size and are uncharged

• Can be exchanged without a change in co-ordination number via ligand substitution reaction

Co-ordination number

• If ligands are different size then the co-ordination number may change

• e.g. Cl- ligands are bigger than the O on H2O, so co-ordination number goes from 6 to 4

Chelate effect

• Positive entropy is favourable in substitution reactions

  • Means more stable complex is being formed

• Achived by substituting unidentate ligands with bidentate or multidentate ligands

  • This is the chelate effect

Increasing the entropy

• Replacing ligands by those that form more co-ordinate bonds

  • e.g. 4 total moles to 7 total moles in equation

• Driven by entropy increase, chelate effect

• Therefore, cannot replace ligands by those that form fewer co-ordination bonds due to entropy decrease

Monodentate ligans

• H2O

• NH3

• Cl-

Substitution may be incomplete e.g.

[Cu(NH3)4(H2O)2]2+

• Occurs with copper complexes and ammonia in excess

Bidentate ligands

• H2NCH2CH2NH2

• C2O4 2-

Multidentate ligands

• EDTA4-

Haemoglobin

• Haem is an iron(II) complex with a multidentate ligand

• There are 4 coordinate bonds around central Fe2+ ion

• Oxygen forms a coordinate bond to Fe(II) in haemoglobin, 4-6 coordinate bonds, enabling oxygen to be ransported in the blood

• CO is a silent killer because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin