3.2.5 Transition metals

• Complex: central metal atom / ion surrounded by ligands
• Ligand: A species which can use its lone pair to form a dative covalent bond to a transition metal.
• Co-ordination number: Number of co-ordinate bonds to the central metal atom/ion
• What shape is formed with a co-ordination number of 6? 

  • Octahedral
  • Small ligands e.g H2O and NH3
• What types of ligands are there?

  
  

  • monodentate = attach with 1 co-ordinate
  • bondbidentate = attach with 2 co-ordinate bonds from two atoms (each donate lone pair)
  • multidentate = attach with multiple co-ordiinate bonds
• What type of isomerism do square planar complexes display? Geometric isomerism
• How can transition metals be identified? by their colour
• Which acid do you use for redox if you want to reduce a species? HCl provides H+ for reduction
• Acid to use for redox if you want to oxidise a species:

  • Use H2SO4 to provide H+ for oxidation
• What is the effect of pH on redox in transition metals? Reduction favoured by acidic conditions
• Bidentate ligand: Coordinate bonds from two atoms
• Transition metals and their compounds can act as hetereogenous and homogenous catalysts
• V2O5 and Fe are examples of heterogenous catalysts
• Catalyst poisoning:

  • Impurities can block active sites
  • so the have reduced efficiency
• What can act as a poison in the Haber process? sulphur
• What can act as a poison in a catalytic converter? Lead
• Haem: An iron (II) complex with a multidentate ligand. 
• CO is toxic because it replaces O2 co-ordinately bonded to Fe (II) in haemoglobin.

  
  

  This prevents Hb from transporting O2
• Chelate effect: Bidentate and multidentate ligands replace monodentate ligands from complexes - resulting in a more stable complex. 
• W is a bad catalyst because it's adsorption is too strong
• Ag is a a bad catalyst because

  
  

  adsorbtion is too weak and reactants do not absorb in high enough concentrations
• Transition element: An element which forms at least one stable ion with a partially full d-shell of electrons
• Where are the transition metals located in the periodic table? In the middle (block from Ti to Cu) - part of the d-block
• What are some characteristic physical properties of transition metals?

  • Metallic
  • good conductors of heat and electricity
  • hard, strong, shiny
  • high m.p., high b.p., low reactivity.
• What are the characteristic chemical properties of transition metals:

  • Variable oxidation states → take part in many redox reactions
  • Coloured compounds/ions in solution
  • Good catalysts
  • Form complex ions
• Catalysts are usually spread over an inert support medium to

  • Increase surface area
  • Reduce cost
• Which electrons do transition metals lose first when forming ions? 4s
• mono/unidentate ligands: A ligand that forms one co-ordinate bond to the central metal ion (one lone pair to donate)
• Give some examples of common monodentate ligands: Cl- , H2O, NH3 , CN
• How many co-ordinate bonds does EDTA4- form? six
• Define the term coordination number: The number of co-ordinate bonds the metal ion has formed to surrounding ligands
• Chelate effect: Chelate complexes with multidentate ligands are favoured over monodentate ligands or ligands that form fewer co-ordinate bonds per molecule
• If a transition metal ion has 2 ligands, what shape is it usually? Linear
• If a transition metal ion has 4 ligands, what shape is it usually? Tetrahedral
• What shape is a complex ion if it has 6 ligands? Octahedral
• What conditions are needed for a complex ion display to optical isomerism? Usually applies to octahedral molecules with 2 or more bidentate ligands, so that the mirror images are non-superimposable
• What affects the colour of a transition metal compound?

  • ΔE affects the frequency of absorbed photons, so determines the colour
  • ΔE is changed by oxidation state of the metal, number and type of ligands, shape, co-ordination number
• What colour is Fe2+’s aqua ion? Green
• What colour is Fe3+’s aqua ion? Pale brown
• What colour is Cr2+’s aqua ion? blue