AQA A Level Chemistry

Revision Notes

6.2.2 Ligand Exchange

Ligand Exchange

  • Ligand exchange (or ligand substitution) is when one ligand in a complex is replaced by another
  • Ligand exchange forms a new complex that is more stable than the original one
  • The ligands in the original complex can be partially or entirely substituted by others
  • There are no changes in coordination number, or the geometry of the complex, if the ligands are of a similar size
  • But, if the ligands are of a different size, for example water ligands and chloride ligands, then a change in coordination number and the geometry of the complex will occur

Substitution in copper(II) complexes

  • When a transition element ion is in solution, it can be assumed that it exists as a hexaaqua complex ion (i.e. it has six water ligands attached to it)
    • For example, Cu2+(aq) is [Cu(H2O)6]2+(aq)
  • The [Cu(H2O)6]2+ (aq) complex ion is blue in colour
  • Upon dropwise addition of sodium hydroxide (NaOH) solution, a light blue precipitate is formed
  • Partial ligand substitution of two water ligands by two hydroxide ligands has occurred

Ligand Exchange equation 1

  • Upon addition of excess concentrated ammonia (NH3) solution, the pale blue precipitate dissolves to form a deep blue solution
  • Again, partial ligand substitution has occurred

Ligand Exchange equation 2

  • If you were to add concentrated ammonia (NH3) solution dropwise to the [Cu(H2O)6]2+ (aq), rather than sodium hydroxide (NaOH) solution, the same light blue precipitate would form
  • Again, the pale blue precipitate will dissolve to form a deep blue solution, if excess ammonia solution is then added

Chemistry of Transition Elements - Ligand Exchange in Copper, downloadable AS & A Level Chemistry revision notes

Water ligands are exchanged by hydroxide and ammonia ligands in the copper(II)  complex

  • The water ligands in [Cu(H2O)6]2+ can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid (HCl)
  • The complete substitution of the water ligands causes the blue solution to turn yellow

Ligand Exchange equation 3

  • The coordination number has changed from 6 to 4, because the chloride ligands are larger than the water ligands, so only 4 will fit around the central metal ion
  • The geometry of the complex has also changed from octahedral to tetrahedral
  • This is a reversible reaction, and some of the [Cu(H2O)6]2+ complex ion will still be present in the solution
    • The mixture of blue and yellow solutions in the reaction mixture will give it a green colour
  • Adding water to the solution will cause the chloride ligands to be displaced by the water molecules, and the [Cu(H2O)6]2+ (aq) ion and blue solution will return

Water ligands are exchanged by chloride ligands in the copper(II) complex

Substitution in cobalt(II) complexes

  • The [Co(H2O)6]2+(aq) complex ion is pink in colour
  • Upon dropwise addition of sodium hydroxide (NaOH) solution, a blue precipitate is formed
  • Partial ligand substitution of two water ligands by two hydroxide (OH) ligands has occurred
    • If the alkali is added in excess, the blue precipitate will turn red when warmed

Ligand Exchange equation 4

  • If excess concentrated ammonia solution is added to [Co(H2O)6]2+, a brown solution will also be formed
    • There will be no precipitate formed in this instance, as the ammonia has been added in excess and not dropwise
  • Complete ligand substitution of the water ligands by ammonia ligands has occurred

Ligand Exchange equation 5

  • The ammonia ligands make the cobalt(II) ion so unstable that it readily gets oxidised in air to cobalt(III), [Co(NH3)6]3+

Chemistry of Transition Elements - Ligand Exchange in Cobalt, downloadable AS & A Level Chemistry revision notes

Water ligands are exchanged by hydroxide and ammonia ligands in the cobalt(II) complex

  • The water ligands in [Co[H2O)6]2+ can also be substituted by chloride ligands, upon addition of concentrated hydrochloric acid
  • The complete substitution of the water ligands causes the pink solution to turn blue

Ligand Exchange equation 2

  • Like with [Cu(H2O)6]2+ above, the coordination number has changed from 6 to 4, because the chloride ligands are larger than the water ligands, so only 4 will fit around the central metal ion
  • The geometry of the complex has also changed from octahedral to tetrahedral
  • Adding water to the solution will cause the chloride ligands to be displaced by the water molecules, and the [Co(H2O)6]2+ (aq) ion and pink solution will return

Chemistry of Transition Elements - Ligand Exchange in Cobalt 2_1, downloadable AS & A Level Chemistry revision notes

Water ligands are exchanged by chloride ligands in the cobalt(II) complex

Incomplete Exchange

  • Transition element ions can form complexes which consist of a central metal ion and ligands
  • Copper(II) and cobalt(II) ions will be used as examples of the central metal ions, in the complex formation with water (H2O), ammonia (NH3), hydroxide (OH), and chloride (Cl) ligands
    • A copper(II) ion has an electronic configuration of 1s22s22p63s23p63d9
    • A cobalt(II) ion has an electronic configuration of 1s22s22p63s23p63d7

Complexes with water & ammonia molecules

  • Water and ammonia molecules are examples of neutral ligands
  • Both ligands contain a lone pair of electrons which can be used to form a dative covalent bond with the central metal ion
    • In water, this is the lone pair on the oxygen atom
    • In ammonia, it is the lone pair on the nitrogen atom
  • Since water and ammonia are small ligands, 6 of them can fit around a central metal ion, each donating a lone pair of electrons, forming 6 dative bonds
    • The coordination number of a complex is the number of dative bonds formed between the central metal ion and the ligands
    • Since there are 6 dative bonds, the coordination number for the complex is 6
  • Complexes with a coordination number of 6 have an octahedral shape
  • The overall charge of a complex is the sum of the charge on the central metal ion, and the charges on each of the ligands
  • A complex with cobalt(II) or copper(II) as a central metal ion, and water or ammonia molecules as ligands, will have an overall charge of 2+
    • The central metal ion has a 2+ charge and the ligands are neutral

Chemistry of Transition Elements - Ammonia and Water Complexes, downloadable AS & A Level Chemistry revision notes

Cobalt(II) and copper(II) form octahedral complexes with ammonia and water ligands

Complexes with hydroxide & chloride ions

  • Hydroxide and chloride ions are examples of negatively charged ligands
  • Both ligands contain a lone pair of electrons which can be used to form a dative covalent bond with the central metal ion
  • Hydroxide ligands are small, so 6 of them can fit around a central metal ion and the complex formed will have a coordination number of 6
  • Chloride ligands are large ligands, so only 4 of them will fit around a central metal ion
  • Complexes with 4 chloride ligands will have a coordination number of 4
    • Complexes with 4 chloride ligands will form tetrahedral complexes
    • Whereas hydroxide ligands will form octahedral complexes
  • A complex with cobalt(II) or copper(II) as a central metal ion and chloride ions as ligands, will have an overall charge of 2-
    • The central metal ion has a charge of 2+
    • Each chloride ligand has a charge of 1-
    • There are 4 chloride ligands in the complex, so the overall negative charge is 4-
    • The overall positive charge is 2+
    • Therefore, the overall charge of the complex is 2-
  • A complex with cobalt(II) or copper(II) as a central metal ion and hydroxide ions as ligands, will have no overall charge
    • The central metal ion has a charge of 2+
    • Each hydroxide ligand has a charge of 1-
    • There are 2 hydroxide ligands in the complex, so the overall negative charge is 2-
    • The overall positive charge is 2+
    • Therefore, the overall charge on the complex is 0

Chemistry of Transition Elements - Chloride and Hydroxide Complexes, downloadable AS & A Level Chemistry revision notes

Cobalt(II) and copper(II) form tetrahedral complexes with chloride and octahedral complexes with water and hydroxide ligands

The Haem Complex

The Haem Complex

The Chelate Effect

  • Different ligands can form different numbers of dative bonds to the central metal ion in a complex.
    • Some ligands can form one dative bond to the central metal ion
    • Other ligands can form two dative bonds, and some can form multiple dative bonds

Monodentate ligands

  • Monodentate ligands can form only one dative bond to the central metal ion
  • Examples of monodentate ligands are:
    • Water (H2O) molecules
    • Ammonia (NH3) molecules
    • Chloride (Cl) ions
    • Cyanide (CN) ions

Chemistry of Transition Elements - Monodentate Ligands, downloadable AS & A Level Chemistry revision notes

Examples of complexes with monodentate ligands

Bidentate ligands

  • Bidentate ligands can each form two dative bonds to the central metal ion
  • This is because each ligand contains two atoms with lone pairs of electrons
  • Examples of bidentate ligands are:
    • 1,2-iaminoethane (H2NCH2CH2NH2) which is also written as ‘en’
    • Ethanedioate ion (C2O42- ) which is sometimes written as ‘ox’

 

Chemistry of Transition Elements - Bidentate Ligands, downloadable AS & A Level Chemistry revision notes

Examples of complexes with bidentate ligands

Polydentate ligands

  • Some ligands contain more than two atoms with lone pairs of electrons
  • These ligands can form more than two dative bonds to the and are said to be polydentate ligands
  • An example of a polydentate ligand is EDTA4- , which is a hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion

 

Chemistry of Transition Elements - Polydentate Ligands_1, downloadable AS & A Level Chemistry revision notes

Example of a polydentate ligand complex

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