Transition elements

• Transition elements


  Elements in the d-block of the periodic table that form one or more stable ions with a partially filled d-orbital
• Transition metals


  • Lose electrons to form positive ions, with the s-orbital electrons being removed first
  • Have similar physical properties including similar atomic radius, high densities and high melting points
  • Have special chemical properties: form complexes, form coloured ions in solution, have variable oxidation states, are good catalysts
• Transition metals have variable oxidation states because the 4s and 3d orbitals are very close in energy levels
• Transition metal elements


  • Scandium
  • Nickel
  • Iron
  • Vanadium
• Copper and chromium are exceptions to the rule that the 4s subshell is filled before the 3d subshell
• Chromium electron configuration


  The actual electron configuration is [Ar] 4s^1 3d^5, not [Ar] 4s^2 3d^4, because it is more stable with one unpaired electron in each 3d orbital
• Copper electron configuration


  The actual electron configuration is [Ar] 4s^1 3d^10, not [Ar] 4s^2 3d^9, because it is more stable with the 3d subshell completely filled
• Catalysts


  • Some transition metal elements and compounds display catalytic behaviour
  • Can be used in industrial manufacture of chemicals
  • Cu^2+ is used as a catalyst for the reaction of Zn with acids, MnO2 is used as a catalyst for decomposition of H2O2
  • Beneficial to use catalysts in industry to reduce energy usage, but increased risk from toxicity of many transition metals
• Complexes

  Transition metals form complexes, consisting of a central metal ion surrounded by ligands
• Ligands


  Molecules or ions with a lone electron pair that can form a dative (coordinate) bond to the central metal ion by donation of this electron pair
• Common ligands

  • Cl-
  • H2O
  • NH3
• Monodentate ligands


  Form one coordinate bond
• Bidentate ligands


  Form two coordinate bonds
• Hexadentate ligands


  Form six coordinate bonds
• Coordination number


  The total number of coordinate bonds formed with the central metal ion
• Coloured ions


  Transition metal ions can be identified by their colour, which changes depending on the coordination number of the complex, the type of ligand bonded to the ion and the oxidation state
• Colour arises because of how substances absorb and reflect light
• Colourless ions


  Ions and complexes where there are no available electrons to excite cannot absorb light and are therefore colourless
• Octahedral complexes


  Transition metal complexes with H2O and NH3 ligands commonly form octahedral complexes with a bond angle of 90°
• Tetrahedral complexes


  Complexes form with larger ligands such as Cl- in a tetrahedral shape with a bond angle of 109.5° and 4 ligands coordinated
• Square planar complexes


  Platinum and nickel complexes form in a square planar shape with four coordinate bonds and a bond angle of 90°
• Cisplatin


  The cis isomer of a square planar complex of platinum, with both chlorine atoms on the same side, commonly used as a cancer therapy drug
• Cisplatin can cause serious side effects such as hair loss, so it has to be administered in small amounts
• Optical isomerism


  Seen in some octahedral complexes with bidentate ligands, where the complexes have non-superimposable mirror images
• Ligand substitution


  1. Ligands in a transition metal complex can be exchanged for other ligands

  2. Substitution with larger ligands like Cl- can change the coordination number of the complex
• Haem


  A component of haemoglobin, consisting of a central Fe^2+ ion and a tetradentate porphyrin ring, with the Fe^2+ ion able to form coordinate bonds with one or two additional axial ligands, giving an overall octahedral structure
• Ligand substitution occurs when the oxygen usually bound to haem is replaced with carbon monoxide, which binds more strongly and is toxic to humans as it prevents oxygen transport
• Chelate effect


  In ligand substitution reactions, a positive entropy change is favourable as it means a more stable complex is being formed, so it is favourable to substitute monodentate ligands with bidentate or polydentate ligands
• e2+


  Can also form coordinate bonds with one or two additional axial ligands
• Haemoglobin


  • Has an overall octahedral structure
  • The shape and structure allow it to transport oxygen around the body
• Ligand substitution reaction


  • Oxygen usually bound to haem is replaced with carbon monoxide
  • Carbon monoxide binds more strongly than oxygen so can not be removed
  • Carbon monoxide is toxic to humans as it prevents oxygen from being transported around the body
• Chelate effect


  • In ligand substitution reactions, a positive entropy change is favourable as it means a more stable complex is being formed
  • It is favourable to have more moles on the right of the reaction than on the left
  • This is achieved by substituting monodentate ligands with bidentate or polydentate ligands
• Enthalpy change


  • For ligand substitution reactions is very small as the bonds being formed are very similar to the bonds that were broken
  • Overall enthalpy change is near to zero
• Hydrated


  Metal ions become hydrated in water when H2O ligands form around the central metal ion
• Metal aqua ions to be known

  • Chromium(III)
  • Iron(II)
  • Iron(III)
  • Manganese(II)
  • Copper(II)
• Reactions of metal aqua ions with NaOH

  1. Metal-aqua ion

  2. With OH-

  3. With excess OH-
• Reactions of metal aqua ions with NH3

  1. Metal-aqua ion

  2. With NH3

  3. With excess NH3
• The number of OH- substituted is the same as the value of the charge on the initial ion
• Variable oxidation states


  One of the key chemical properties of transition metals
• Redox reactions


  • Used to change the oxidation state of a transition metal
  • Often accompanied by a colour change