Hard And Soft Acids/Bases

Question 1

Which factors will ligand displacement depend on

Answer 1

1. The nature of the Ligand/other ligands
2. The nature of the acceptor, Metal

Since some ligands form stronger complexes with metals than others

Question 2

What are hard acids and soft acids

Answer 2

Hard acids are:
1. Non-polarisable cations
2. Small radius
3. High effective nuclear charge (charge to radius ratio)
4. High energy LUMO
E.g. Most metals in an oxidation state, H+, électropositive metals

Soft acids:
1. More polarisable cations
2. Larger radius
3. Lower charge to radius ratio
4. Lower energy LUMO than hard acids
E.g. Cu(I), Ag(I), Au(I), etc. All d-block metals in

Question 3

What are hard and soft bases

Answer 3

Hard bases:
1. Small
2. Non-polarisable
3. Electronegative
4. Difficult to oxidise
5. Low energy HOMO
E.g. F-, OH-, NH2, NO2-, SO42-, CO32-

Soft bases
1. Big
2. Polarisable
3. Easier to oxidise
4. Higher energy HOMO
E.g. Br-, I- S, P

Question 4

Give an example of ambidentate ligands in relation to hard and soft bases

Answer 4

Thiocyanate coordinates through Sulphur, which is soft, and isothiocyanate coordinates through N, which is hard

Question 5

How do hard and soft acids/bases influence complex stability

Answer 5

1. Complex with hard acid and base is most stable due to electrostatic interactions
2. Complex with soft acid and soft base is most stable due to good HOMO LUMO interactions leading to strong covalent bonds