Lecture 4

Question 1

What is the hard and soft acid and base (HSAB) concept?

Answer 1

• developed by Ralph Pearson, can be used to predict coordination preferences
• Metal ions are electron pair acceptors: Lewis acids
  Ligands are electron pair donors: Lewis bases
• Important criteria: Polarisability
  Charge density (or charge-to-size ratio)
  Nature of the bonding (ionic vs. more covalent)

Question 2

Polarisability

Answer 2

Susceptibility ligand donor atoms or metal ions to experience a charge shift in their electron shell:
- hard centres: are little affected
- soft centres: are easily polarisable
Affinities: high between centres of the same type
hard/hard: highly ionic bond
soft/soft: partly covalent bond
- useful qualitative concept to predict; the stability of complexes

Question 3

What are hard/soft electron pair donors?

Answer 3

• hard electron-pair donors: F-, R-O-
• soft electron-pair donors: R-S-, S2-
• borderline: Nitrogen: hard when –NH2
  but borderline in pyridine (aromatic ring
  makes it more polarisable

Question 4

Which TM ions are classed as hard/soft?

Answer 4

Metal ions which form their most stable complexes with hard oxygen donors are class a or hard.
Examples: Sc3+, Ti4+, VO2+, Cr3+, Mn2+, Fe3+
usually highly charged and small

Metal ions which form their strongest complexes with soft sulfur or phosphorus donors are class b or soft.
Examples: Cu+, all d-block metals in zero oxidation state
larger, low charge density and more
polarisable

Borderline metal ions have no clear preferences e.g. Fe2+, Co2+, Ni2+, Cu2+

Question 5

How does Gold mining occur using TM properties?

Answer 5

Soft Au+ has a high affinity for soft CN-: [Au(CN)2]- is so stable that gold (which is normally very difficult to oxidise) can be oxidised by air:
4Au + 8CN- + O2 + 2H2O = 4[Au(CN)2]- + 4OH-

This oxidation reaction is used to extract elemental gold from large volumes of ore. the gold is then isolated via reduction.
Large scale processes are of environmental concern