15.9 - Ligand Exchange Reactions

• what are ligand exchange reactions?

  when one ligand is swapped for another ligand

  reversible reaction
• what happens during a ligand exchange reaction?

  could change in coordination number if ligands have different sizes or charges so changes shape of complex ion leading to a colour change
• why are ligand exchange reactions reversible?

  ligands can be replaced easily depending on the concentration of ligands present
• splitting effect of octahedral complexes

  greater than tetrahedral complexes
• why is enthalpy change for ligand exchange usually small?

  dative covalent bonds are broken and formed so strength of bonds broken is very similar to strength of bonds formed
• what's entropy?

  a measure of disorder or randomness in a system
• high entropy

  more particles

  more likely reaction is to occur
• substituting monodentate ligands with bidentate/multidentate ligands

  increases entropy (more positive) due to more particles so complexes with bidentate/multidentate ligands are more stable
• K(stab) - stability constant

  equilibrium constant for the formation of complex ions in aqueous solutions
• larger Kstab

  more stable complex ion compared with that containing water
• expression for Kstab

  same as the one for Kc

  don't include water as it is constant
• what's haemoglobin?

  a iron (II) complex with a multidentate ligand

  4 nitrogen atoms in a porphyrin ring form 4 dative covalent bonds with Fe2+
• reaction of haemoglobin with oxygen/water

  reversible ligand substitution reaction
• carbon monoxide poisoning

  ligand exchange reaction where oxygen is replaced by carbon monoxide

  CO forms strong covalent bonds with Fe2+ in an irreversible reaction so haemoglobin can no longer transport oxygen
• ammonia + copper sulfate solution - type of reaction

  ligand exchange reaction

  ammonia molecules are exchanged for water molecules around Cu2+
• ammonia + copper sulfate solution - colour changes

  from pale blue solution to pale blue precipitate when ammonia isn't in excess

  when ammonia is in excess pale blue precipitate forms a deep blue solution
• ammonia + copper sulfate solution - change in coordination number

  stays the same because ligands are both uncharged and are similar in size
• HCl + copper sulfate solution - type of reaction

  ligand exchange reaction where Cl- replaces water around Cu2+
• HCl + copper sulfate solution - colour change

  from pale blue to green if there's not enough Cl-, if it's in excess it turns yellow
• HCl + copper sulfate solution - change in coordination number

  changes from 6 to 4 because Cl- ions are larger than water molecules so less can fit around central Cu2+ ion
• HCl + cobalt salt solution - type of reaction

  ligand exchange reaction where Cl- replaces water
• HCl + cobalt salt solution - colour change

  pale pink to deep blue
• HCl + cobalt salt solution - change in coordination number

  from 6 to 4 as Cl- is larger than water so less can fit around central ion

  shape changes from octahedral to tetrahedral