17A5-6. Transition metals

• Square planar complexes


  Also formed by transition metal ions, cis-platin is an example which is used in cancer treatment and supplied as a single isomer, not a mixture with the trans form
• Xenon tetrafluoride (XeF4)


  • Xenon is a noble gas but forms some stable compounds, has 8 electrons in its outermost energy level, each F atom uses 1 electron to form a covalent bond, resulting in 12 electrons in the outer energy shell arranged in 6 pairs in an octahedral arrangement, with 2 lone pairs opposite each other and the 4 bonding pairs in a plane with F-Xe-F bond angle of 90°
• cis-platin


  A square planar complex of platinum(II) with two ammonia and two chloride ligands, used as an effective treatment for some types of cancer
• cis-trans isomers


  Identical ligands are next to each other in cis, opposite each other in trans
• cis-platin can form a bond between the two strands of DNA, preventing them from separating and preventing cancer cell division
• trans-platin is a much less effective cancer treatment than cis-platin and more toxic
• Only the cis- isomer of platin should be used in cancer treatment
• Monodentate ligands


  Use one lone pair of electrons to form a dative bond, e.g. H2O, Cl-, NH3
• Bidentate ligands


  Use two lone pairs of electrons to form dative bonds, e.g. NH2CH2CH2NH2 (ethylenediamine)
• Multidentate/Polydentate ligands


  Use multiple lone pairs of electrons to form dative bonds, e.g. EDTA4- (ethylenediaminetetraacetic acid)
• Haemoglobin is an iron(II) complex containing a polydentate ligand, and ligand exchange occurs when oxygen bound to haemoglobin is replaced by carbon monoxide
• Substitution of a monodentate ligand by a bidentate or hexadentate ligand leads to a more stable complex ion due to a positive increase in the ΔS system
• Multidentate ligand


  An organic ion with a rather complicated structure that forms multiple dative bonds with a metal ion
• 1,2-diaminoethane


  Also known as ethylenediamine, a molecule with two nitrogen atoms that can form two dative bonds with a metal ion
• EDTA


  Ethylenediaminetetraacetic acid, a hexadentate ligand that forms six dative bonds with a metal ion
• EDTA- is the ion formed when each of the ethanoic acid groups in EDTA loses its H+ ion, giving an ion with four negative charges
• Stability of complexes


  Refers to the comparison of stabilities of two complexes where the number of ligands has changed, not the stability of complexes with transition metal ions in unstable oxidation states
• Ligand exchange reaction


  1. Monodentate ligand replaced by bidentate or hexadentate ligand

  2. Increases total number of species

  3. Increases disorder and entropy

  4. Favours formation of more stable products
• Haemoglobin is a protein in red blood cells that plays a vital role in transporting oxygen through the bloodstream
• Haemoglobin structure


  • Consists of protein (globin) and four haem groups
  • Each haem group has four nitrogen atoms that hold an Fe2+ ion in a square planar structure
  • There is a fifth dative bond from the protein to the Fe2+ ion
• Oxygen transport by haemoglobin

  1. Oxygen molecules act as ligands, forming dative bonds with Fe2+ ions in haem groups

  2. Oxygen is transported to cells and then released
• Haemoglobin + oxygen


  Oxyhaemoglobin
• Haemoglobin + carbon monoxide

  Carboxyhaemoglobin
• Carbon monoxide forms a much stronger dative bond with haemoglobin than oxygen, replacing oxygen and reducing oxygen transport
• Monodentate ligand: a molecule or ion that forms one dative bond with a metal ion
• Bidentate ligand: a molecule or ion that forms two dative bonds with a metal ion
• Multidentate ligand: a molecule or ion that forms several dative bonds with a metal ion
• Hexadentate ligand: a molecule or ion that forms six dative bonds with a metal ion