1. Multidentate Ligands

2. Denticity Denticity – comes from the Latin word ‘dentis’ meaning tooth
Monodentate ligands – think like ‘one tooth’ or ‘one bite’
Ligand only using one pair of electrons on the atom to form the dative bond with the metal ion.
So what would bidentate mean?
What about multidentate?

3. Complex Formation [Cu(H2O)6]2+ [CuCl4]2-
“Dentate” – Latin for “tooth” ...
How often they can bite the metal ion
Unidentate ligands – form one co-ordinate bond
e.g. H2O:, :OH-, :NH3, :CN-, :Cl-
[Cu(H2O)6]2+
[CuCl4]2-
Draw out the Lewis structures (dot-and cross diagrams) of the ligands as well
Show each co-ordinate bond as an arrow pointing from the ligand to the central metal ion

4. Complex Formation [Cr(C2O4)3]3- [Cr(en)3]3+
Bidentate ligands – form two co-ordinate bonds
ethanedioate
(C2O42-)
1,2-diaminoethane
(en)
[Cr(C2O4)3]3-
[Cr(en)3]3+
Can donate 2 pairsof e- to the central metal ion in a complex
Common ligands: 1,2-diaminoethane
Ethanedioate ion

5. Complex Formation
Bidentate ligands – form two co-ordinate bonds
1,2-diaminoethane
(en)
How would you show the reaction between hexaaquachromium (III) and 1,2-diaminoethane?
[Cr(en)3]3+
Can donate 2 pairsof e- to the central metal ion in a complex
Common ligands: 1,2-diaminoethane
Ethanedioate ion

6. Complex Formation e.g. [Cu(EDTA)]2-
Multidentate ligands – form several co-ordinate bonds
EDTA4-
e.g. [Cu(EDTA)]2-
EDTA4- can donate 6 pairs of e- to the metal ion.
2 lone pairs are on its N atoms and 4 are on the O on the carboxylate groups.
EDTA can be used (under medical supervision) as antidote to poisoning by TM (usually lead poisoning)
Chelated ions are excreted in urine
My supervisor at St. Andrews fed EDTA to his son who had swallowed some TM compound that was in his house! Think it was lead.

7. Chelation:
Complex ions with polydentate ligands are called “Chelates”
Greek for “Claw”
Chelates can be used to effectively remove d-block metal ions from solution

8. Stability of complexes
Comparison of the stabilities of two complexes in which the number of ligands has changed.
[Cu(H2O)6]2+(aq) + 3en- (aq) → [Cu(en)3]2+(aq)+ 6H2O(l)
The total number of species has increased from four to seven
The system is more disordered
Increase in Δsystem
Increase in stability of the products compared to the reactants, so formation of products is favoured.

9. [Cu(H2O)6]2+ (aq) + EDTA4- (aq) → [CuEDTA]2-(aq)+ 6H2O(l)
The Chelate Effect
[Cu(H2O)6]2+ (aq) + EDTA4- (aq) → [CuEDTA]2-(aq)+ 6H2O(l)
2 species replaced by 7
Increase in number of particles causes a significant increase in ENTROPY
A LOT more stable
Drives reaction to right

10. What do the following have in common?
Molecule of month article on EDTA
RSC article on uses of EDTA

11. Complex Formation
Multidentate ligands – form several co-ordinate bonds
e.g. haem
Discuss haemoglobin here
See notes in textbook p103
From SPEC:
Haem is an iron(II) complex with a multidentate ligand. Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood. Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin.

12. What happens if O2 becomes CO?
Ligand substitution reaction
Haemoglobin + oxygen  REVERSIBLE
Haemoglobin + carbon monoxide  IRREVERSIBLE

14. Summary Question
Write an equation for the ligand substitution reaction that occurs when four ligands in the hexaaquairon(III) ion are replaced by 1,2-diaminoethane (en).

15. Exam Questions