Substitution reactions

What occurs in a ligand substitution reaction?

One ligand is replaced by another

What 2 ligands are a similar size?

• NH₃

• H₂O

[Co(H₂O)₆]²⁺ + 6NH₃ ⇌

(Excess ammonia)

[Co(NH₃)₆]²⁺ + 6H₂O

What is the colour, co-ordination number and shape of the metal complex before and after this reaction?
[Co(H₂O)₆]²⁺ + 6NH₃ ⇌ [Co(NH₃)₆]²⁺ + 6H₂O

Before

• Colour: Pink

• Co-ordination number: 6

• Shape: Octahedral

After

• Colour: Pale brown

• Co-ordination number: 6

• Shape: Octahedral

[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌

(Excess ammonia)

[Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

What is the colour, co-ordination number and shape of the metal complex before and after this reaction?
[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

Before

• Colour: Blue

• Co-ordination number: 6

• Shape: Octahedral

After

• Colour: Deep blue

• Co-ordination number: 6

• Shape: Octahedral

What ion is larger than NH₃ and H₂O ligands?

Chloride ion, Cl^-

[Co(H₂O)₆]²⁺ + 4Cl^- ⇌

(Concentrated HCl)

[CoCl₄]^2- + 6H₂O

What is the colour, co-ordination number and shape of the metal complex before and after this reaction?

[Co(H₂O)₆]²⁺ + 4Cl^- ⇌ [CoCl₄]^2- + 6H₂O

Before

• Colour: Pink

• Co-ordination number: 6

• Shape: Octahedral

After

• Colour: Blue

• Co-ordination number: 4

• Shape: Tetrahedral

[Cu(H₂O)₆]²⁺ + 4Cl^- ⇌

(Concentrated HCl)

[CuCl₄]^2- + 6H₂O

What is the colour, co-ordination number and shape of the metal complex before and after this reaction?

[Cu(H₂O)₆]²⁺ + 4Cl^- ⇌ [CuCl₄]^2- + 6H₂O

Before

• Colour: Blue

• Co-ordination number: 6

• Shape: Octahedral

After

• Colour: Yellow

• Co-ordination number: 4

• Shape: Tetrahedral

[Fe(H₂O)₆]³⁺ + 4Cl^- ⇌

(Concentrated HCl)

[FeCl₄]^- + 6H₂O

What is the colour, co-ordination number and shape of the metal complex before and after this reaction?
[Fe(H₂O)₆]³⁺ + 4Cl^- ⇌ [FeCl₄]^- + 6H₂O

Before

• Colour: Pale Yellow

• Co-ordination number: 6

• Shape: Octahedral

After

• Colour: Deeper Yellow

• Co-ordination number: 4

• Shape: Tetrahedral

What is the colour, co-ordination number and shape of the metal complex before and after this reaction?
[Fe(H₂O)₆]²⁺ + 4Cl^- ⇌ [FeCl₄]^2- + 6H₂O

Before

• Colour: Pale green

• Co-ordination number: 6

• Shape: Octahedral

After

• Colour: Pale green

• Co-ordination number: 4

• Shape: Tetrahedral

Haem

An iron(II) complex with a multidentate ligand

What is the co-ordination number of iron(II) in the haem unit?

4

What is the shape around iron(II) in the haem unit?

Square planar

What is Haem bound to?

A complex protein called globin

What 2 ligands can bind to haem’s Iron(II) in the body?

• H₂O

• O₂

What is the coordination number around the iron in oxyhaemoglobin?

6

Describe what happens to haemoglobin in the lungs

• In lungs concentration of oxygen

• Equilibrium shifts right

• Water ligands are substituted for oxygen

• Oxyhaemoglobin is formed

Describe what happens to oxyhaemoglobin at the respiring tissue

• Oxyhaemoglobin gives up O₂ to a place where it is needed

• Water takes the place of O₂

• Haemoglobin returns back to lungs to start process again

How does Carbon monoxide poisoning occur?

• When carbon monoxide is inhaled, the haemoglobin can substitute water ligands for carbon monoxide

• The bond between carbon monoxide and iron(II) is very strong and not readily reversed

• This prevents oxygen being bonded to haemoglobin, so CO is considered toxic

• Leads to oxygen starvation in organs

Chelate effect

Multidentate and bidentate ligands replace monodentate ligands to form more stable complexes