transition metals

• Transition metals


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

  
• Transition metals


  • They lose electrons to form positive ions, with the s-orbital electrons being removed first
  • They have similar physical properties including atomic radius, high densities and high melting and boiling points
  • They form complexes
  • They form coloured ions in solution
  • They have variable oxidation states
  • They are good catalysts
• Variable oxidation states


  Possible because the 4s and 3d orbitals are very close in energy levels, making it possible for electrons to be lost from both orbitals relatively easily
• Copper and chromium are exceptions to the rule that the 4s sub-shell is filled before the 3d sub-shell
• Chromium electron configuration


  It is more stable if one of the electrons from the 4s orbital is instead in the 3d orbital, so that each 3d orbital contains one unpaired electron. Therefore, the actual electron configuration is [Ar] 4s^1 3d^5.
• Copper electron configuration


  It is more stable if the 3d sub-shell is completely filled. Therefore, one of the 4s electrons is moved to a 3d orbital to give the electron configuration [Ar] 4s^1 3d^10.

  
• Complexes


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


  • [Co(NH3)6]^3+
  • [Co(H2O)6]^3+
• Ligands


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

  
• Silver complexes have a coordination number of 2 and platinum complexes commonly have a coordination number of 4

  
• 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

  
• Aqueous solutions of transition metal ions tend to be coloured
• Colour


  Arises because of how substances absorb and reflect light. When white light shines on a substance, some of the wavelengths of light are absorbed, but the remaining wavelengths are reflected and transmitted to the human eye. These reflected wavelengths correspond to a specific colour which is then observed by the human eye.

  
• Colour in transition metal complexes

  Ligands cause the d-orbital to split, meaning some electrons exist in a slightly higher energy level (in an 'excited state'). The change in energy (ΔE) between these states corresponds to a specific wavelength and frequency of light.
• Some metal ions and complexes are colourless. Colour arises when some wavelengths of light are absorbed while others are reflected, which involves the excitation of electrons. Ions and complexes where there are no available electrons to excite and move around 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°. Octahedral complexes are formed when there is six-fold coordination with monodentate ligands.

  
• Tetrahedral complexes


  When complexes form with larger ligands such as Cl-, they form tetrahedral complexes with a bond angle of 109.5°. Tetrahedral complexes can show optical isomerism.

  
• Square planar complexes


  Platinum and nickel complexes form in a square planar shape. This consists of four coordinate bonds with a bond angle of 90°.

  
• Cisplatin


  • The cis isomer of a square planar complex of platinum, commonly used as a cancer therapy drug
• Cisplatin can cause serious side effects such as hair loss, meaning it has to be administered in small amounts to try and reduce these effects.
• Ligand substitution


  Ligands in a transition metal complex can be exchanged for other ligands
• Haem


  • A component of haemoglobin, a common multidentate ligand consisting of six coordinate bonds around a central Fe^2+ ion. Its shape and structure allow it to transport oxygen around the body.
• Ligand substitution occurs when the oxygen usually bound to haem is replaced with carbon monoxide, which is toxic to humans as it prevents oxygen from being transported around the body.
• Vanadium oxidation states


  Vanadium has four possible oxidation states from +5 to +2 each of which produces a different coloured compound
• The phrase "You Better Get Vanadium" can be used to remember the colours of the different vanadium oxidation states
• Reduction of vanadium


  Vanadium can be reduced from an oxidation state of +5 all the way to +2 via reduction with zinc in acidic conditions
• Reduction of vanadium


  The E-cell values show that reduction becomes less favourable as the oxidation state of vanadium decreases, to the point where the oxidation of V2+ to V3+ is more favourable than the reduction of V3+ to V2+
• Role of pH


  The pH of the reaction conditions determines whether a transition metal is oxidised or reduced. Acidic conditions are required for ions to be reduced, while alkaline conditions are required for ions to be oxidised.
• Chromium oxidation states


  The most stable oxidation states of chromium are +6, +3 and +2, found in the species Cr2O7^2-, Cr3+ and Cr2+ respectively. These can be interconverted by oxidation and reduction reactions.

  
• Oxidation and reduction of chromium

  1. Cr2O7^2- can be reduced to Cr3+ and Cr2+ ions by reduction with zinc in acidic conditions

  2. Cr3+ can be oxidised to Cr2O7^2- by hydrogen peroxide in alkaline conditions, first forming CrO4^2- which is then converted to Cr2O7^2- by acidification
• Equilibrium between chromium(VI) species

  There is an equilibrium between the two chromium(VI)-containing species 2CrO4^2- + 2H+ ⇌ Cr2O7^2- + H2O. Acidification of a solution containing CrO4^2- will cause the equilibrium to shift to the right, increasing the concentration of Cr2O7^2-.
• Amphoteric


  Metal hydroxides that can react with both acids and bases
• Reaction of chromium hydroxide with acid

  Chromium hydroxide precipitate reacts with acid
• Reaction of chromium hydroxide with base

  Chromium hydroxide precipitate reacts with base
• Ligand substitution reactions


  Ligands in a transition metal complex can be exchanged for other ligands
• Copper and cobalt aqua ions can undergo substitution with chloride ions</b>