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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

2.5 Transition Metals (A-level only)

2.5.1 General Properties of Transition Metals 2.5.2 Substitution Reactions 2.5.3 Shapes of Complex Ions 2.5.4 Formation of Coloured Ions 2.5.5 Variable Oxidation States 2.5.6 Catalysts

Ligand Substitution Reactions

Specification Reference Inorganic chemistry, Transition metals 3.2.5.2

Quick Notes

  • H2O, NH3, and Cl− are monodentate ligands — they can donate a single pair of electrons to a central metal ion to form a co-ordinate bond.
    • NH3 and H2O are similar in size and uncharged, so ligand exchange between them does not change the co-ordination number or charge of the complex.
    • Cl− is larger than H2O and NH3, so substitution of H2O by Cl− often leads to a change in co-ordination number from 6 to 4.
  • Ligands can also be bidentate (form two co-ordinate bonds) (e.g., H2NCH2CH2NH2, C2O42−) or multidentate (form more than two co-ordinate bonds) (e.g., EDTA4−).
  • The chelate effect describes how bidentate or multidentate ligands tend to replace monodentate ligands due to an increase in entropy.
  • Haem is an iron(II), Fe2+ complex with a multidentate ligand and oxygen (O2) can form a co-ordinate bond to the Fe(II) in haemoglobin, enabling it to be transported in the blood.
  • Carbon monoxide (CO) is toxic because it binds more strongly to the iron(II) in haemoglobin than oxygen, preventing oxygen transport.

Full Notes

Ligands in a complex ion can sometimes be substituted for different ligands in what are called ligand substitution or ligand exchange reactions.

Monodentate Ligands (H2O, NH3, and Cl−)

Monodentate ligands can donate one lone pair to the central metal ion.

For Example: Common examples include H2O (aqua), NH3 (ammine) and Cl− (chloride)

AQA A-Level Chemistry examples of monodentate ligands H2O, NH3, and Cl−

NH3 and H2O are similar in size and uncharged, and usually six molecules of each can fit around a central ion in a complex, getting close enough to form co-ordinate bonds to it.

This gives the complex a co-ordination number of 6 and an octahedral shape.

AQA A-Level Chemistry shapes of complexes with H2O or NH3 (octahedral, CN = 6) and with Cl− (tetrahedral, CN = 4)

Chloride ions, Cl−, are larger than H2O and NH3, and only four Cl− ligands can fit around a central ion, giving the complex a co-ordination number of 4 and a tetrahedral shape.

Substitution of H2O by NH3

Only six H2O or NH3 ligands can get close enough to a metal ion to co-ordinately bond to it.

As a result, if ligand substitution occurs and H2O ligands are substituted or exchanged for NH3 ligands, there is no change in co-ordination number (stays 6).

For Example Cu2+ complex with NH3:

[Cu(H2O)6]2+ + 4NH3 ⇌ [Cu(NH3)4(H2O)2]2+ + 4H2O

Colour change: Blue → Deep Blue.

Substitution is incomplete (only 4 NH3 molecules replace H2O) however co-ordination number remains 6.

Substitution of H2O by Cl−

Chloride (Cl−) ligands are larger than H2O, so the co-ordination number decreases (from 6 to 4) if substitution occurs.

This is because only four Cl− ligands can get close enough to the central metal ion to co-ordinately bond to it.

For Example Cu2+ complex with Cl-:

[Cu(H2O)6]2+ + 4Cl− ⇌ [CuCl4]2− + 6H2O

Colour change: Blue → Yellow and co-ordination number changes from 6 to 4.

Bidentate and Multidentate Ligands

Bidentate ligands form two co-ordinate bonds.

For Example
Ethyl-1,2-diamine (H2NCH2CH2NH2) and ethanedioate (C2O42−) .

AQA A-Level Chemistry examples of bidentate ligands such as ethane-1,2-diamine and ethanedioate

Multidentate ligands form multiple co-ordinate bonds.

For Example
EDTA4− (forms 6 co-ordinate bonds, replacing all H2O ligands).

AQA A-Level Chemistry diagram of multidentate ligand EDTA4− showing multiple donor sites

The Chelate Effect

There is a tendency for bidentate or multidentate ligands to replace monodentate ligands in complexes.

This is driven by an increase in entropy and is called the Chelate Effect.

For Example: Reaction of [Cu(H2O)6]2+ with C2O42− ions

AQA A-Level Chemistry chelation example forming [Cu(C2O4)3]4− from [Cu(H2O)6]2+ and ethanedioate (C2O4^2-) ions

Entropy increases (+ΔS) as the number of free particles increases (4 reactant particles compared to 7 product particles). ΔG is more negative, making the reaction more feasible.

Haemoglobin and Oxygen Transport

Haemoglobin in red blood cells contains Fe2+ bound to a multidentate ligand (haem). O2 binds to Fe2+ via a co-ordinate bond, allowing oxygen transport in the blood. Caron Monoxide (CO) is toxic because it binds more strongly to the Fe2+ than O2.

AQA A-Level Chemistry schematic of haem group showing Fe2+ centre and multidentate ligand environment

This binding of CO instead of O2 decreases the ability of blood to transport oxygen:

AQA A-Level Chemistry comparison of O2 vs CO binding to the Fe2+ centre in haemoglobin

Summary