2A1 Transition metals

• A transition metal is one with an incomplete d-subshell in its atoms or ions
• Anomalous transition metal electron configurations

  1. Chromium: prefers to have a half-full s and d subshell. six outer electrons occupy orbitals singly so there is no repulsion
  2. Copper: prefers to have a full d subshell than a full s subshell
• ions formed from transition metals lose their 4s electrons first to form cations
• Properties of transition metal:

  1. form complex ions
  2. form coloured ions
  3. have variable oxidation states
  4. show catalytic activity
• A complex ion contains a central metal atom or ion surrounded by ligands which are bonded coordinately
• ligands are molecules or ions with an available lone pair of electrons
• The coordination number of a complex is the total number of coordinate bonds between the metal and the ligands
• Common ligands:

  • H2O and NH3 which are neutral
  • Cl- and OH- which are anions
• Copper sulfate

  • when pure it is a white solid and anhydrous
  • if water is added to it creates an exothermic reaction
  • forms hexaaquacopper(II) ion [Cu(H2O)6]2+
• Ligands with only one lone pair of electrons available are unidentate
• Ligands are bidentate if they form 2 coordinate bonds with metal ions
• Mutlidentate ligands

  • ligands with many lone pairs on a variety of atoms
  • e.g. EDTA 4- has six donor atoms with an available lone pair
  • titration calculations with EDTA 4- have a ratio of 1:1
• Complexes formed between multidentate ligands and metals are chelates and are more stable than complexes with unidentate ligands
• Haem

  • red ion (II) complex found in blood
  • four of the six coordination points in the structure are taken by N atoms
  • the fifth position is taken up by the protein globin
  • last position is taken up by a weakly bonded oxygen atom
• Complex ion shapes: octahedral

  • six ligands surround a metal
  • e.g. [Cu(H2O)6]2+
  • coordination number is 6
• Complex ion shapes: tetrahedral

  • coordination number is 4
  • e.g. [CuCl4]2-
• Complex ion shapes: square planar

  • coordination number of 4
  • e.g. cis-platin
  • Cl- ligand is larger than H2O and NH3 so fewer can fit around the metal
  • Cl- ligand is negative so will cause repulsion between neighbouring anions
• Complex ion shapes: linear

  • coordination number is 2
  • very common in silver (I) and copper (I) complexes
• Colour is associated with incomplete d subshells in transition metal ions
• In order for a colour change to occur there must be a change in:

  • oxidation state of the metal
  • coordination number of the complex
  • type of ligand involved
• Change in oxidation state

  • [Fe(H2O)6]2+ --> [Fe(H2O)6]3+
  • green --> brown
• Change in coordination number

  • [Cu(H2O)6]2+ + 4Cl- --> [CuCl4]2- + 6H2O
  • blue --> green
• Change of ligand

  • [Cu(H2O)6]2+ + 4NH3 --> [Cu(NH3)4(H2O)]2+ + 4H2O
  • blue --> deep blue
• The origin of colour in a transition metal ion is in the d subshell; if it has an electron configuration d0 or d10 it is colourless
• Colour changes are measured by looking at intensity changes which mirror metal ion concentration and are performed by visible spectrophotometer or colourimeter
• Colourimeter

  • light of various wavelengths are passed through the solution which hits a photosensitive detector producing an electrical current
  • the amount of light let in is proportional to the ion concentrations
  • a calibration curve is drawn
• Some metal ion complexes produce a pale colour. Other ligands can be added to make the colour more intense and easier to measure

  • e.g. Cu (II) with EDTA produces a more intense blue
• transition metal ions have more than one oxidation state so undergo redox reactions and change from one state to another
• Zinc

  • good reducing agent
  • in acidic solution will reduce most transition metal ions from higher to lower oxidation states
  • Zn (s) --> Zn2+ (aq) +2e-
• Vanadium

  • ammonium vanadate (V), NH4VO3 is a white solid
  • if added to dil HCl it dissolves to form an orange solution due to the presence of dioxovanadium(V) ion VO2 +
  • addition of zinc results in a gradual colour change in vanadium
  • to achieve its final state, a stopper must be added to exclude air
  • using sulphuric acid instead of HCl will give duller colours
• Complex: VO2 +

  Oxidation state: +5

  Colour: orange
• Complex: VO2+

  Oxidation state: +4

  Colour: blue
• Complex: V(H2O)6 3+

  Oxidation state: +3

  Colour: green
• Complex: V(H2O)6 2+

  Oxidation state: +2

  Colour: violet
• Chromium

  • maximum oxidation state of +6
  • seen in chromate and dichromate
  • in HCl dichromate (VI) ions can be redcued by zinc metal
  • Cr(II) is easily oxidized by oxygen so a flask needed to be stopped loosley
  • [Cr(H2O)6]3+ is ruby coloured but is difficult to see so ligands are usually substituted and a green complex is seen
• Complex: Cr2O7 2-

  Oxidation state: +6

  Colour: orange
• Complex: Cr 3+ (aq)

  Oxidation state: +3

  Colour: green
• Complex: Cr 2+ (aq)

  Oxidation state: +2

  Colour: blue
• Manganese

  • oxidation state of +7
  • common ion manganate (VII) is purple in solution
  • when reduced to [Mn(h2O)6]2+ a pale pink colour appears
• Redox titrations

  • KMnO4- potassium manganate
  • K2Cr2O7- potassium dichromate
  • Both need acids conditions to work to measure various chemicals in solution (e.g. Fe2+)