Ligand substitution and the chelate effectAQA A-Level Chemistry: Revision notes
Section 1
Monodentate ligands and ligand substitution
H₂O, NH₃ and Cl⁻ are monodentate ligands: each donates one lone pair and forms one co-ordinate bond. In ligand substitution one ligand in a complex is replaced by another, for example:
[Co(H₂O)₆]²⁺ + 6NH₃ → [Co(NH₃)₆]²⁺ + 6H₂O
The central ion keeps its oxidation state, so this is not a redox reaction.
Section 2
Substitution without change of co-ordination number
NH₃ and H₂O are similar in size and uncharged, so exchange of these ligands happens without a change of co-ordination number. Cobalt(II) and copper(II) both stay six co-ordinate.
Substitution may be incomplete. With copper, excess ammonia gives [Cu(NH₃)₄(H₂O)₂]²⁺ and not [Cu(NH₃)₆]²⁺:
[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
The pale blue solution turns deep blue.
Balance the number of water molecules released with the number of ammonia ligands added: four NH₃ in, four H₂O out.
Section 3
Substitution with change of co-ordination number
The Cl⁻ ligand is larger than NH₃ and H₂O, so fewer Cl⁻ ions fit around the metal ion. Exchange of H₂O by Cl⁻ can therefore change the co-ordination number from 6 to 4:
[Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O [Fe(H₂O)₆]³⁺ + 4Cl⁻ ⇌ [FeCl₄]⁻ + 6H₂O
Check the charge: the complex charge is the metal ion charge plus the ligand charges. Adding water shifts these equilibria back to the hexaaqua ions.
Six water molecules are released when four chloride ions are taken up, because all six water ligands are lost.
Section 4
Bidentate and multidentate ligands
A bidentate ligand donates two pairs of electrons and forms two co-ordinate bonds, for example ethane-1,2-diamine, H₂NCH₂CH₂NH₂ (en), through its two N atoms, and the ethanedioate ion, C₂O₄²⁻, through two O atoms. [Ni(en)₃]²⁺ has a co-ordination number of 6.
EDTA⁴⁻ is a multidentate (hexadentate) ligand and forms six co-ordinate bonds, so one EDTA⁴⁻ ion fills all six sites in an octahedral complex.
The co-ordination number is the number of co-ordinate bonds, not the number of ligands.
Section 5
Haemoglobin and carbon monoxide
Haem is an iron(II) complex with a multidentate ligand. In haemoglobin, oxygen forms a co-ordinate bond to Fe(II), so oxygen is carried round the blood. The bond is reversible, so oxygen is released at the tissues.
Carbon monoxide is toxic because it replaces the oxygen co-ordinately bonded to Fe(II). It forms a stronger bond than oxygen and is not easily displaced, so the haemoglobin can no longer carry oxygen.
The bond from oxygen to iron is a co-ordinate bond, not an ionic bond, and the O₂ molecule donates the pair of electrons.
Section 6
The chelate effect
Bidentate and multidentate ligands replace monodentate ligands, and the products are more stable. This is the chelate effect.
[Ni(NH₃)₆]²⁺ + 3en → [Ni(en)₃]²⁺ + 6NH₃
The number of particles rises from 4 to 7, so the entropy increases. The enthalpy change is about zero because the same number and type of bonds (six Ni–N) are broken and made. Since ΔG = ΔH − TΔS, ΔG is negative and the reaction is feasible.
Worked example: ΔH = −34 kJ mol⁻¹ and ΔS = +245 J K⁻¹ mol⁻¹ at 298 K. TΔS = 298 × 245 = 73 010 J = 73.0 kJ mol⁻¹, so ΔG = −34 − 73.0 = −107 kJ mol⁻¹.
Convert ΔS from J to kJ (or ΔH to J) before combining it with ΔH in ΔG = ΔH − TΔS.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ligand substitution and the chelate effect
- Aqueous copper(II) sulfate contains the pale blue complex ion [Cu(H₂O)₆]²⁺. When an excess of concentrated ammonia solution is added the colour changes to deep blue, but the product is [Cu(NH₃)₄(H₂O)₂]²⁺ rather than [Cu(NH₃)₆]²⁺.Explain why the co-ordination number of copper does not change when ammonia replaces water in this reaction.2 marks
- A student investigates reactions of aqueous metal ions with concentrated hydrochloric acid. Aqueous copper(II) sulfate contains [Cu(H₂O)₆]²⁺ and aqueous iron(III) nitrate contains [Fe(H₂O)₆]³⁺. Each reacts with the acid to form a yellow-green solution of [CuCl₄]²⁻ and a yellow solution of [FeCl₄]⁻ respectively.Write an equation for the formation of [CuCl₄]²⁻ from [Cu(H₂O)₆]²⁺ and state the change in co-ordination number of copper.2 marks
- Nickel(II) ions in aqueous solution form [Ni(H₂O)₆]²⁺, and adding excess ammonia solution gives [Ni(NH₃)₆]²⁺. When ethane-1,2-diamine, H₂NCH₂CH₂NH₂ (en), is added, the ammonia ligands are replaced: [Ni(NH₃)₆]²⁺ + 3en → [Ni(en)₃]²⁺ + 6NH₃. This reaction goes essentially to completion at room temperature.Explain why en is described as a bidentate ligand and state the co-ordination number of nickel in [Ni(en)₃]²⁺.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).