6.2.2 Ligand Exchange Flashcards
1
Q
What happens during ligand exchange?
A
- One ligand is partially or entirely substituted for another
- Forms a more stable complex
- Overall charges can change (if ligand charge changes)
- Coordination number can change (if size of ligands is changed)
- Geometry of complex ion can change (if ligands have different sizes)
2
Q
Why might incomplete ligand substitution happen?
A
- If energetics of reaction are not favourable
- If stability of product is not favourable
- Different sized ligands can lead to incomplete substitution
- Copper (II) illustrates this with ammonia
3
Q
What happens when NaOH is added dropwise to a solution containing Cu(H2O)6 2+ ions?
A
- Solution is pale blue due to Cu(H2O)6 2+ ions
- Light blue precipitate forms due to Cu(OH)2(H2O)4 present
- Partial ligand substitution occurred of 2 OH ligands
4
Q
What happens if excess concentrated ammonia is added to Cu(OH)2(H2O)4?
A
- From light blue
- To deep blue solution
- Partial ligand substitution
- Cu(OH)2(H2O)4
- Into Cu(NH3)4(H2O)2 2+
5
Q
What happens if concentrated ammonia is added dropwise to to Cu(H2O)6 2+?
A
- Light blue solution
- (if excess NH3 added, deep blue solution forms again)
6
Q
What happens when HCL is added to Cu(H2O)6 2+?
A
- Solution turns from pale blue (due to Cu(H2O)6 2+
- To yellow
- Due to CuCl4 2-
- Complete ligand substitution of chloride ligands
- Coordination number changed from 6 to 4
- Cl ligands larger than H2O ligands so only 4 fit around central metal ion
- Mixture of Cu(H2O)6 2+ and CuCl4 2- solution can cause green colour from blue + yellow solution
- Adding water can displace Cl ligands and blue solution returns
7
Q
What is haemoglobin structurally?
A
- Multidentate ligand
- Fe 2+ as central metal ion
- Oxygen ligands form coordinate bonds to Fe 2+
- Weak ligands with weak bonds so can dissociate easily to be transported to cells
8
Q
Why is carbon monoxide toxic in the blood?
A
- Better ligand than oxygen
- Binds irreversibly to haemoglobin
- So oxygen cannot be carried to cells
9
Q
What is the chelate effect?
A
- Replacement of monodentate ligands with bidentate and multidentate ligands
- negative ΔG, so entropy drives this reaction
- Entropy change always positive because reaction produces more particles
10
Q
How does the chelate effect work only in terms of balancing entropy and enthalpy change?
A
- Small enthalpy change required
- Relatively large positive entropy change required
- For ΔG to be negative
