C23 the transition metals

• a transition element is an element that forms at least one stable ion with a partially full d-shell of electrons
• transition metals are the elements from titanium to copper, which are in the d-block of the periodic table
• the transition elements are metals, so are good conductors of heat and electricity, hard, strong, shiny, have high melting and boiling points, and are reasonably unreactive
• transition metals have 2 outer 4s sub-level electrons, and as you go across the period, electrons are added to the inner 3d sub-level, which makes the elements all have similar properties
• chromium and copper do not fit the pattern, they both have only one electron in the d sub-level, which we assume makes them more stable
• transition metals have four main common properties:

  • variable oxidation states
  • colour
  • catalysis
  • complex formation
• transition metals have variable oxidation states, this means they have more than one oxidation state in their compounds, so can take part in many different redox reactions
• transition metals produce a wide variety of coloured metalions
• many transition metals show catalytic activity
• transition metals form complex ions, which is a metal ion surrounded by ligands joined by coordinate bonds
• all transition metal ions can form coordinate bonds by accepting electron pairs from other molecules, which are called ligands
• a ligand is an ion or molecule with a lone pair of electrons that forms a co-ordinate bond with a transition metal
• examples of ligands are:

  • H2O:
  • :NH3
  • :Cl-
  • :CN-
  • C2O4(2-)
  • H2NCH2CH2NH2
  • EDTA(4-)
• sometimes, two, four, or six ligands bond to a single transition metal ion, this forms a complex ion
• the number of co-ordinate bonds to ligands that surround the d-block metal ion is called the co-ordination number
• typically, ions with a coordination number of 6 form octahedral shapes and ions with a coordination number of 4 form tetrahedral shapes, but some ions with a coordination number of 4 form square planar shapes
• monodentate ligands have one atom which can donate a single pair of electrons
• bidentate ligands have two atoms which can donate a pair of electrons each
• multidentate ligands have multiple atoms which can donate a pair of electrons each
• examples of monodentate ligands are H2O, NH3 and Cl-
• examples of bidentate ligands are H2NCH2CH2NH2 and C2O4(2-)
• examples of multidentate ligands are EDTA(4-)
• the chelate effect is that multidentate and bidentate ligands will displace monodentate ligands in a complex
• the chelate effect works because the reaction has a positive entropy change as a more stable complex is formed, and the greater the entropy change, the more negative the Gibbs free energy is, so the reaction is more favourable
• most transition metal compounds are coloured, because they absorb energy in the visible section of the spectrum
• delocalised excited d-orbital electrons move between d-orbitals as they absorb energy, this energy is often in the visible section of the spectrum, the frequencies that are absorbed are missing from the spectrum when you view it, therefore the colour you view depends on the reflected frequencies
• the frequency of light is related to the energy by the equation E=hf
• in E=hf:

  E = energy in J

  h = planck's constant

  f = frequency in Hz
• you can test colour using a colorimeter, which measures the absorbance of visible light by passing light through a sample onto a detector
• vanadium has 4 oxidation states from +5 to +2, each produces a different colour, these compounds are formed by the reduction of vanadate(V) ions by zinc in acidic solution
• the oxidation states of vanadium are:

  VO2(+) = +5 = yellow

  VO(2+) = +4 = blue

  V(3+) = +3 = green

  V(2+) = +2 = purple
• the pH of reaction conditions determines whether a transition metal is oxidised or reduced
• for transition metal ions to be reduced, the conditions must be acidic
• for transition metal ions to be oxidised, the conditions must be alkaline
• the reduction of Tollen's reagent to metallic silver is used to distinguish between aldehydes and ketones
• redox titrations are limited to three main reactions:

  • MnO4(-) + 8H+ + 5e- -> Mn(2+) + 4H2O
  • Fe(2+) -> Fe(3+) + e-
  • C2O4(2-) -> 2CO2 + 2e-
• the ratio of ${MnO_4}^-$to ${C_2O_4}^{2-}$is 2:5
• the ratio of ${MnO_4}^-$to $Fe^{2+}$is 1:5
• the ratio of ${C_2O_4}^{2-}$to $Fe^{2+}$is 1:2
• transition metals and their compounds can act as heterogeneous and homogeneous catalysts and many are used for this in industry