Multidentate Ligands

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?

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

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

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

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.

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

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.

[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

What do the following have in common? Molecule of month article on EDTA http://www.chm.bris.ac.uk/motm/edta/edtah.htm RSC article on uses of EDTA http://www.rsc.org/chemistryworld/podcast/CIIEcompounds/transcripts/EDTA.asp

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.

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

https://www.youtube.com/watch?v=c9hvRhZE9kw

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).

Exam Questions