Transition Metal Complexes (Cambridge (CIE) A Level Chemistry): Revision Note

Complex formation

What is a complex?

• A complex is a molecule or ion formed by a central metal atom or ion surrounded by one or more ligands

• A ligand is a species with a lone pair of electrons that can be donated to the metal ion

  • Ligands form dative covalent bonds with the metal by donating their lone pair of electrons

• For example, [Al(H₂O)₆]³⁺ (aq):

Transition metal complex formation

• Transition metal ions readily form complexes with ligands

• Copper(II) and cobalt(II) ions will be used as examples of the central metal ions, in the complexes with:

  • Water (H₂O)

  • Ammonia (NH₃)

  • Hydroxide (OH^-)

  • Chloride (Cl^-)

Co(II) and Cu(II) complexes with water & ammonia

• Water and ammonia are neutral ligands

  • Water donates a lone pair from the oxygen atom

  • Ammonia donates a lone pair from the nitrogen atom

• Water and ammonia are small ligands

  • Up to 6 water or ammonia ligands can fit around a central metal ion

  • This results in 6 dative covalent bonds

• 6 dative covalent bonds give:

  • An octahedral shape

  • A coordination number of 6

    • The coordination number of a complex is the number of dative covalent bonds formed between the central metal ion and the ligands

Cobalt(II) and copper(II) complexes with water and ammonia

• 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

• For a cobalt(II) or copper(II) complex with 6 water or ammonia ligands:

  • The central metal ion has a charge of 2+

  • The ligands have a charge of 0

  • So, the overall charge of the complex is (2+) + (6 x 0) = 2+

Complexes with hydroxide & chloride ions

• Hydroxide ions, OH^-, and chloride ions, Cl^-, are negatively charged ligands

  • Each donates a lone pair of electrons to form a dative covalent bond with the central metal ion

Hydroxide complexes

• Hydroxide ions are small ligands

  • Up to 6 hydroxide ions can fit around a central metal ion

  • This results in:

    • 6 dative covalent bonds

    • An octahedral shape

    • A coordination number of 6

Chloride complexes

• Chloride ions are large ligands

  • Up to 4 chloride ligands can fit around a central metal ion

  • This results in 4 dative covalent bonds

• 4 dative covalent bonds give:

  • A tetrahedral shape

  • A coordination number of 4

Charges of Co(II) complexes with hydroxide and chloride ligands

• Co(II) ions commonly form complexes with 2 hydroxide ion ligands

  • The remaining ligands are water

• For this cobalt(II) complex:

  • The central metal ion has a charge of 2+

  • The water ligands have a charge of 0

  • The 2 hydroxide ligands have a charge of 2 x (-1) = -2

  • So, the overall charge of the complex is (2+) + (4 x 0) + (-2) = 0

[Co(H₂O)₄(OH)₂]

• Co(II) ions commonly form complexes with 4 chloride ion ligands

• For this cobalt(II) complex:

  • The central metal ion has a charge of 2+

  • The 4 chloride ligands have a charge of 4 x (-1) = -4

  • So, the overall charge of the complex is (2+) + (-4) = -2

[CoCl₄]^2-

Charges of Cu(II) complexes with hydroxide and chloride ligands

• Cu(II) ions commonly form complexes with 2 hydroxide ion ligands

  • The remaining ligands are water

• For this copper(II) complex:

  • The central metal ion has a charge of 2+

  • The water ligands have a charge of 0

  • The 2 hydroxide ligands have a charge of 2 x (-1) = -2

  • So, the overall charge of the complex is (2+) + (4 x 0) + (-2) = 0

[Cu(H₂O)₄(OH)₂]

• Cu(II) ions commonly form complexes with 4 chloride ion ligands

• For this copper(II) complex:

  • The central metal ion has a charge of 2+

  • The 4 chloride ligands have a charge of 4 x (-1) = -4

  • So, the overall charge of the complex is (2+) + (-4) = -2

[CuCl₄]^2-

Comparing copper(II) and cobalt(II) complexes with chloride and water / hydroxide ions