Chemical properties of transition metals

• The transition metals have some characteristic chemical properties, including:
  • Forming compounds with variable oxidation states.
  • A strong tendency to form complexes.
  • Forming coloured compounds.
  • Useful as catalysts.
• Much of these properties are brought about by the fact that the transition metals can form variable oxidation states.

Variable oxidation states

• Unlike s-block elements, which are limited to oxidation numbers of +1 (for group 1), or +2 (for group 2), transition metals can form a huge variety of oxidation states.
• This is because of their successive ionisation energies.
• The graph above shows the successive ionisation energies for calcium (s-block metal) and vanadium (transition metal).
• For calcium, the first two electrons removed are from the outer energy level (4s sub-shell), and so the ionisation energy is fairly low. There is a sharp increase in calcium’s ionisation energies after the first two electrons are removed; the electrons become harder to remove (removed from the filled 3p sub-shell).
• However, for vanadium there is a gradual increase in the ionisation energies, as the electrons are first removed from the 4s sub-shell and then the incomplete 3d sub-shell. Removing form the 3d sub-shell doesn’t require much more energy, so a greater number of electrons can be removed.
• The amount of energy required for each successive ionisation energy determines how many electrons can be involved in bonding.
• As can be seen from the previous graph, it doesn’t require much more energy to remove a third electron from vanadium; thus it can form a 3⁺ ion. In higher oxidation states, the ions form covalent bonds with oxygen or fluorine, for example (VO₂)⁺ (v).
• The table below shows the most common oxidation states of some of the transition metals:
• Copper is the only element above in which the +1 ion is important.
• For the rest of the elements, the sum of the first two ionisation energies is low enough for the first two electrons to be removed (excepting scandium, the 2+ ions are formed when the 4s electrons have been lost).
• The first three ionisations are also low enough for three electrons to be removed (except zinc).
• Note that the number of successful ionisation energies increases from Sc to Mn, after which it decreases again.
• This is because from Sc to Mn, the highest oxidation state is simply the sum of the 3d and 4s electrons.
• After Mn, the highest oxidation states become lower and less stable, as more protons in the nucleus result in the positive charge acting upon them being stronger.
• Generally, the elements in simple ionic compounds usually have the lower oxidation states, for example Cu²⁺ in copper sulphate.
• The metals with higher oxidation states are usually bonded covalently to oxygen or fluorine (e.g. MnO₄^- manganate (vii) ion).
• As the change from one oxidation state to another is a redox reaction, it is possible to predict the relative stability of the compound using standard electrode potentials.
• In general,
  • Higher oxidation states become less stable compared to lower ones as you move from left to right across the series.
  • Compounds containing metals in high oxidation states tend to be oxidising agents (e.g. MnO₄^- ).
  • Compounds containing metals in low oxidation states are usually reducing agents.
  • The relative stability of the +2 state compared the +3 state increases across the series (i.e. to the left of the series, the +2 state is highly reducing; whereas to the right of the series, the +2 state is stable, and the +3 state is highly oxidising).

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