Transition metals

• Ligand definition


  A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.
• Monodentate ligands

  H2O, NH3, Cl-
• The ligands NH3 and H2O are similar in size and uncharged
• Exchange of the ligands NH3 and H2O occurs without change of co-ordination number
• Transition metal ions commonly form tetrahedral shaped complexes with larger ligands such as Cl-
• The multidentate ligand EDTA4- has 6 lone pairs
• Haem is an iron(II) complex with a quadridentate ligand. It has a square planar shape.

  There is space for two extra co-ordinate bonds above and below the haem ring, one of which can be formed with H2O, which can be easily replaced by O2.

  CO is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin, but it forms a very stable complex.