Transition Metal Complexes (CIE A Level Chemistry)

• 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 that contains one or more lone pairs of electrons
• The ligand forms a dative covalent bond with the central metal atom or ion, by donating its lone pair of electrons

An example metal complex

This complex is formed from a central aluminium(III) ion and six water molecule ligands coordinately bonded

• 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

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

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
  • Complexes with 6 hydroxide ligands will have a coordination number of 6
  • Complexes with 6 hydroxide ligands will form 
  • octahedral complexes
• 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
• 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

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