Transition Metals: Substitution Reactions

Question 1

What are complexes?

Answer 1

• A complex is a central metal atom or ion surrounded by ligands by dative bonding

Question 2

What is a ligand?

Answer 2

• A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons

Question 3

What is a co-ordination number?

Answer 3

• A co-ordination number is number of co-ordinate bonds to the central metal atom or ion

Question 4

What are the different types of ligands?

Answer 4

• 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

Question 5

Give some examples of monodentate ligands

Answer 5

• Ammonia
• Chloride ions
• Water

Question 6

Give examples of bidentate ligands

Answer 6

• Ethanedioate ions

- Ethane-1,2-diamine

Question 7

Give an example of a multidentate ligand

Answer 7

• EDTA^4-

Question 8

Describe the similarities between ammonia and water

Answer 8

• Both similar in size

- Both uncharged

Question 9

What occurs during a ligand substitution reaction?

Answer 9

• 1 ligand is replaced by another ligand

Question 10

What can be affected by ligand substitution reactions?

Answer 10

• Co-ordination number of complex
• Shape of complex
• Colour of complex
• Oxidation state of metal

Question 11

Substitution of similarly sized ligands

Answer 11

• 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

Question 12

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?

Answer 12

• [Co(H2O)6]^2+(aq) + 6NH3(aq) ⇌ [Co(NH3)6]^2+(aq) + 6H2O(l)

• Ligand
- Water ammonia

• Colour

• Co-ordination number
- 6

• Shape
- Octahedral

Question 13

Substitution of different sized ligands

Answer 13

• 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

Question 14

Which ligand is larger than H2O and NH3?

Answer 14

• Cl- ligand

Question 15

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?

Answer 15

• [Cu(H2O)6]^2+(aq) + 4Cl–(aq) ⇌ [CuCl4]^2–(aq) + 6H2O(l)

• Coordination number
- 6 → 4

• Shape
- Octahedral → tetrahedral

Question 16

Partial substitution of ligands

Answer 16

• Sometimes ligand substitution is only partial

- E.g the formation of [Cu(NH3)4(H2O)2]^2+)

Question 17

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?

Answer 17

• [Cu(H2O)6]^2+(aq) + 4NH3(aq) ⇌
[Cu(NH3)4(H2O)2]^2+(aq) + 4H2O(l)

• Co-ordination number
- 6

• Shape
- Octahedral

• Colour

Question 18

What co-ordination number and shape do complexes with Cl- ligands have?

Answer 18

• Co-ordination number
- 4

• Shape
- Tetrahedral

Question 19

Haem

Answer 19

• 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

Question 20

Chelate effect

Answer 20

• 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