Substitution reaction

• Ligands are classified by the number of dative covalent or coordinate bonds they make
• • Monodentate/Unidentate - Only form one dative bond e.g. H2O, OH-, Cl- and NH3
  • Bidentate - Can form 2 dative bonds e.g. ethane-1,2-diamine (en) as both nitrogen atoms donate lone pairs to metal ions. And ethanedioate (C2O4 2-) as the two single bonded oxygen atoms both donate lone pairs
  • Multidentate - form multiple dative bonds
• Naming bonding atoms in ligands:

  • H2O = Aqua
  • NH3 = Ammine
  • OH- = Hydroxo
  • CN- = Cyano
  • Cl- = Chloro
• Multidentate example is EDTA 4- ions which form 6 coordinate bonds

  
• A ligand substitution reaction occurs when a ligand in a complex ion is replaced by another type of ligand molecules substitution can be by:

  • Of similarly sized ligands ( e.g. 6H2O replaced with 6NH3)
  • Of different sized ligands (e.g. 6H2O replaced with 4Cl-)
  • Partial substitution
• Ligand substitution - different sized

  • When concentrated HCl is added to a solution of hexaaquacopper (||), the chloride ions replace the water molecules as ligands.
  • Equation: [Cu(H2O)6]2+ + 4Cl- ---> [CuCl4]2- + 6H2O
  • The 4H+ ions are spectator ions
  • Since Cl ions are bigger then water only 4 can fit this would change the shape from the octahedral to tetrahedral and the coordination number
  • Colour change from blue to green
• Ligand substitution - similar sized shapes

  • The coordination number and shape doesn't change
  • E.g. [Co(H2O)6]2+ (aq) + 6NH3 (aq) --> [Co(NH3)6]2+ (aq) + 6H2O
  • Colour change form pink to a straw colour
• Ligand substitution - partial substitution

  • Sometimes not all the ligands will be replaced, only some will
  • E.g. [Cu(H2O)6]2+ (aq) + 4NH3 --> [Cu(NH3)4(H2O)2]2+ (aq) +4H2O
  • Colour change from pale blue to deep blue
  • The shape changes from octahedral to elongated octahedral
• Haemoglobin carries oxygen from lungs to cell

  • Contains an Fe2+ ion which forms a haem complex with a tetradentate ligand
  • It also binds to a unidentate globin molecule
  • It has one coordination site left that can bind loosely to an oxygen molecules and carbon dioxide
  • When in an area of low concentration of oxygen , the oxygen would be released
• Carbon monoxide can also strongly binds to haemoglobin which would stop oxygen from bonding as it irreversibly destroys haemoglobins ability to carry oxygen

  • More CO = less O2 going to cells = Death that's why carbon monoxide is toxic
• During ligands exchange, bonds are broken and formed. The strength of both types of bonds is often very similar so the enthalpy change is usually very small

  • E.g. [Ni(NH3)6]2+ (aq) + 3NH2CH2CH2NH2 ---> [Ni(NH2CH2CH2NH2)3] 2+ (aq) + 6NH3 ∆H = -13kj
• Complex ions containing multidentate ligands such as EDTA are called chelates. They are far more stable then complex ions with unidentate ligands. As they:

  • Stops the reaction being reversed easily
  • The stability is known as the chelate effect
• The chelate effect is because of the effect of entropy.

  • When a multidentate ligand replaces unidentate ligands in a complex, it releases many molecules which increases entropy
  • The reverse reaction involves a large decrease in entropy which is why it is so unfavourable
  • E.g. [M(H2O)6]2+(aq) + EDTA4-(aq) ---> [M(EDTA)]2- (aq) + 6H2O
  • So on the left side 2 species is formed but on the right side 7 species are formed so the forward reaction is more favourable
  • More species = More ∆S = More stable = More likely to form
• The enthalpy change (∆H) for ligand substitution reactions is very small as the bonds being formed are very similar to the bonds that were broken. As well as the number of bonds formed and broken are the same. Therefore overall enthalpy change is near zero.
• The greater the entropy change, the more negative the free energy change (∆G) will be, therefore meaning the reaction is more favourable.