Transition Metals: Substitution Reactions Flashcards
What are complexes?
- A complex is a central metal atom or ion surrounded by ligands by dative bonding
What is a ligand?
- A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons
What is a co-ordination number?
- A co-ordination number is number of co-ordinate bonds to the central metal atom or ion
What are the different types of ligands?
• Monodentate
- Ligands that only form 1 co-ordinate bond
• Multidentate
- Ligands that form at least 2 co-ordinate bonds
• Bidentate
- Multidentate ligands that can form 2 co-ordinate bonds
Give some examples of monodentate ligands
- Ammonia
- Chloride ions
- Water
Give examples of bidentate ligands
- Ethanedioate ions
- Ethane-1,2-diamine
Give an example of a multidentate ligand
- EDTA^4-
Describe the similarities between ammonia and water
- Both similar in size
- Both uncharged
What occurs during a ligand substitution reaction?
- 1 ligand is replaced by another ligand
What can be affected by ligand substitution reactions?
- Co-ordination number of complex
- Shape of complex
- Colour of complex
- Oxidation state of metal
Substitution of similarly sized ligands
- If ligands are of similar size (e.g H2O and NH3) then co-ordination number of complex ion and shape stays same
- In case of H2O and NH3, colour still changes due to change in ligand
Write an equation for the ligand substitution reaction that occurs when hexaaquacobalt(II), [Co(H2O)6)]2+ reacts with concentrated ammonia. What are the changes?
• [Co(H2O)6]^2+(aq) + 6NH3(aq) ⇌ [Co(NH3)6]^2+(aq) + 6H2O(l)
• Ligand
- Water ammonia
• Colour
• Co-ordination number
- 6
• Shape
- Octahedral
Substitution of different sized ligands
- If ligands are of different sizes (e.g Cl- is larger than H2O and NH3) there’s a change in co-ordination number and shape
Which ligand is larger than H2O and NH3?
- Cl- ligand
Write an equation for the ligand substitution reaction that occurs when hexaaquacopper(II), [Cu(H2O)6)]^2+ reacts with concentrated hydrochloric acid. What are the changes?
• [Cu(H2O)6]^2+(aq) + 4Cl–(aq) ⇌ [CuCl4]^2–(aq) + 6H2O(l)
• Coordination number
- 6 → 4
• Shape
- Octahedral → tetrahedral
Partial substitution of ligands
- Sometimes ligand substitution is only partial
- E.g the formation of [Cu(NH3)4(H2O)2]^2+)
Write an equation for the ligand substitution reaction that occurs when hexaaquacopper(II), [Cu(H2O)6]2+ reacts with concentrated ammonia. What are the changes?
• [Cu(H2O)6]^2+(aq) + 4NH3(aq) ⇌
[Cu(NH3)4(H2O)2]^2+(aq) + 4H2O(l)
• Co-ordination number
- 6
• Shape
- Octahedral
• Colour
What co-ordination number and shape do complexes with Cl- ligands have?
• Co-ordination number
- 4
• Shape
- Tetrahedral
Haem
- Haem is an iron(II) complex with a multidentate ligand
• Haemoglobin
- Fe(II) in haemoglobin forms 6 co-ordinate bonds
- 4 nitrogen atoms in a porphyrin ring form co-ordinate bonds with iron, making up the haem
- 1 nitrogen atom in protein globin forms co-ordinate bond with iron
• Oxygen and water
- Both bind to iron as ligands
- Both molecules form weak co-ordinate bonds with iron
• Carbon monoxide
- Carbon monoxide forms a strong bond with iron and doesn’t readily exchange with water or oxygen, making carbon monoxide toxic
Chelate effect
- Bidentate and multidentate ligands replace monodentate ligands from complexes, increasing number of particles and therefore result in increase in entropy
- The greater the △S, the more negative the free energy charge (△G) will be, therefore the reaction is more favourable
- The enthalpy change (△H) for ligand substitutions is very small as the bonds being formed are very similar to broken bonds so overall
