Transition Metal Complexes (Cambridge International A-Level Chemistry)

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):

Al(III) is the central metal ion with 6 water ligands, each donating a lone pair

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

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

• 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

Exam Hint Although up to 6 hydroxide ions can fit around a central metal ion, in many examples only 2 hydroxide ions are present alongside 4 water ligands

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

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