Transition metals

• Transition metals


  Elements in the d-block of the periodic table with a partially filled d-orbital of electrons
• Transition metals


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


  Ability of transition metals to exist in different oxidation states
• The energy levels of the 4s and 3d sublevels are very close to one another in transition metals
• This means that different numbers of electrons can be gained or lost using similar amounts of energy
• Oxidation state


  The number of electrons lost by an atom in a chemical reaction
• The oxidation state of a transition metal ion depends on the energetics of the reaction
• Most stable oxidation states of transition metals

  • Chromium: +2, +3, +6
  • Manganese: +2, +4, +7
  • Iron: +2, +3
  • Cobalt: +2, +3
  • Copper: +1, +2

  
• Different oxidation states of transition metals can produce different coloured aqueous solutions
• Complexes


  Consist of a central metal ion or atom surrounded by coordinately bonded ligands
• Ligands


  Molecules, atoms or ions which donate a pair of electrons to a central transition metal ion to form a coordinate bond
• Coordination number


  The number of coordinate bonds formed around the central metal ion

  
• Common coordination numbers are 6 and 4
• Small ligands like NH3 or H2O have a coordination number of 6, while larger ligands like Cl- have a coordination number of 4
• Some silver complexes have a coordination number of 2 and a linear shape
• Octahedral complexes


  • Transition metal complexes with H2O and NH3 ligands commonly form octahedral complexes with a bond angle of 90°
  • The ligands don't all have to be the same, but they must be of a similar size to maintain the octahedral shape

  
• Tetrahedral complexes


  • Complexes form with larger ligands such as Cl- and have a bond angle of 109.5°
• Coloured compounds


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


  Higher energy state that electrons in the d-orbital can move to when given energy
• The change in energy (ΔE) between ground state and excited state corresponds to a wavelength and frequency of light
• The repulsion from the ligands' electrons and the electrons in the d orbitals of the metal ion increases the energy of the d orbitals, but the energies are not all raised by the same amount, splitting the orbitals into groups
• Different ligands on the metal complex lead to different splittings of the d orbitals, which is why the different complexes display different colours
• Octahedral compounds


  [Cu(H2O)6]2+ - pale blue solution

  [Cu(NH3)4(H2O)2]2+ - deep blue solution

  [Co(H2O)6]2+ - pink solution
• Tetrahedral compounds


  [CuCl4]2- - olive green solution

  [CoCl4]2- - deep blue solution

  
• Ligand substitution reaction


  One ligand can be swapped for another ligand without a change in coordination number

  With copper complexes and NH3 in excess, the substitution is incomplete and the complex formed has a combination of water and ammonia ligands

  Substitution with the larger Cl- ligand results in a change in coordination number
• Ligand substitution reactions take place when concentrated HCl is reacted with the metal complex

  
• Ligand exchange


  Substitution reaction where a ligand is replaced by another ligand
• NH3 and H2O ligands

  • Similar in size
  • Uncharged
• NH3 and H2O ligands

  Can be exchanged without a change in coordination number via a ligand substitution reaction
• Ligand substitution reaction


  • Solution changes colour from pink to yellow straw colour
• With copper complexes and NH3 in excess, this substitution is incomplete and the complex formed has a combination of water and ammonia ligands

  
• Ligand substitution reaction with copper complexes

  • Solution changes colour from pale blue to deep blue

  
• Cl- ligand


  • Much larger than NH3 and H2O ligands
  • Substitution with this ligand results in a change in coordination number for that complex
• Ligand substitution reaction with concentrated HCl


  • Solution changes colour from pale blue to yellow
  • Solution changes colour from pink to blue
• Complexes with just Cl- ligands always have a coordination number of four, producing a tetrahedral shaped complex
• Catalyst


  • A substance that speeds up the rate of a reaction without being used up in the reaction
  • Provides an alternative reaction pathway with a lower activation energy
  • Doesn't affect the position of equilibrium but allows it to be reached faster
• Heterogeneous catalysts


  • Catalysts that are in a different phase or state to the species in the reaction
• Heterogeneous catalysts


  • Solid iron catalyst used in the Haber Process
  • Nickel catalyst used in the hydrogenation of alkenes

  
• Adsorption


  1. Solid catalyst works by adsorbing molecules onto an active site on the surface of the catalyst

  2. Active sites increase the proximity of molecules and weaken the covalent bonds in the molecules so that reactions occur more easily and rate of reaction is increased