A2-Level Transition Metals

Transition Metals

  • Transition metals form at least one stable ion with a partially filled d-subshell

  • Even though not all transition metals behave identically, most share the following properties:

    • High melting and boiling points (compared to S-block metals)

    • Relatively low reactivity (compared to S-block metals)

    • Multiple oxidation states

    • Form complex ions

    • Form coloured compounds

    • Can act as catalysts

Colour and Transition Metals

  • The d-orbitals in a transition metal atom can be split into high energy and low energy orbitals. This creates an energy gaps between the two sets of orbitals.

  • Electrons from lower energy d-orbitals can be excited to higher energy d-orbitals by absorbing energy.

  • White light contains waves of electromagnetic radiation that have different energies and colour.

    • If the energy required to excite an electron can be found in white light, specific wavelengths of light are absorbed by the transition metal and are removed from the white light.

    • The complimentary colour of the wavelengths absorbed is now observed and the transition metal has a ‘colour’.

d-orbital Splitting

  • There are five d-shaped orbitals that each occupy a different area in space around the nucleus of an atom

  • Ligands have lone pairs of electrons that repel electrons in some d-orbitals more than others as they move towards to a metal ion

  • D-orbitals have different shapes, meaning some orbitals are repelled more by approaching ligands than others – making them higher energy and the other orbitals lower energy.

  • The d-orbitals are split into two groups – high energy and low energy, with a specific energy gap between them.


  • Coloured solutions absorb specific wavelengths of visible light. The more concentrated a solution, the greater the intensity of light absorbed.

  • Colorimetry is used to find the concentration of a solution based upon the intensity of light (of a specific wavelength) it absorbs.

    • The amount of light that passes through the sample is detected and this gives an indication as to the amount of light absorbed by the sample.

    • A calibration curve is made using the absorbance’s of solutions of known concentrations.

    • The absorbance of the unknown solution is compared to the calibration curve and the concentration found. ​

Redox Titrations

  • Titrations can be used to find the unknown concentration of a solution.

  • Redox reactions consist of a species that is oxidised and a species that is reduced.

    • If the moles of one species is known, using the ratios of the oxidised to species to reduced species, the moles of the other species can be found and the concentration calculated. ​


  • Transition metals have variable oxidation states, enabling them to form ions with different charges.

    • These ions can lose or gain electrons to form another stable ion than can then gain or lose electrons to reform the original ion.

    • In a reaction, this means a transition metal can provide an alternative route and allow a species to be oxidized and then another species to be reduced, whilst remaining unchanged overall.

    • This means the transition metal is acting as a catalyst.

Heterogeneous Catalysis

  • In heterogeneous catalysis, the catalyst is in a different phase to the reactants.

  • With solid based catalysts and gaseous or aqueous reactants, the catalysis process occurs in stages:

    • Reactants diffuse onto catalyst surface.

    • The reactants are absorbed onto the catalyst.

    • The reaction occurs and products made.

    • Products desorb (leave) the surface of the catalyst.

Homogeneous Catalysis

  • In homogeneous catalysis, a catalyst is in the same phase as the reactants.

  • Aqueous transition metal ions are a common example of homogeneous catalysts. They have variable oxidation states and can easily oxidise or reduce other aqueous ions.

A2-Level Complex Ion Chemistry

Complex Ion Chemistry

  • A positive metal ion in water is surrounded by water molecules.

  • The slightly negative oxygen atom in a water molecule forms a co-ordinate (dative) bond with the metal ion.

  • A species that forms a co-ordinate bond with a positive metal ion is called a ligand.

  • Only six water molecules can get close enough to a metal ion to form co-ordinate bonds, creating a complex ion with a co-ordination number of six. 

  • Co-ordination number refers to how many co-ordinate bonds there are to the metal ion.

  • If there are only water ligands in a complex ion, the complex ion’s overall charge is the same as the metal ion in the complex, as water molecules have no overall charge.

Metal Aqua-Ion Reactions

  • Metal aqua complex ions can behave as (very) weak acids in solution.

  • High polarization of the O-H bond in water ligands enables a proton to be removed by a water molecule, and the ligand becomes a negatively charged OH group. 

  • Equilibriums are established for the removal of a proton from the aqua complex ion.

  • Adding hydroxide ions forces the position of equilibriums to favour the release of protons by the complex ions.

  • If the number of OH groups becomes the same as the positive charge of the metal ion, the complex ion becomes a solid precipitate, as it can no longer be dissolved in solution.

  • The higher the positive charge of the metal ion, the more acidic the complex ion.

Ligand Substitution

  • Ligands in a metal complex ion can be substituted.

  • Ligands that have a similar size can be substituted with no change to the co-ordination number of the complex ion.

  • If the ligands involved in substitution are different sizes, a change in co-ordination number can occur.

  • Chloride ions are larger ligands than water molecules so only four can fit around a metal ion – a change in co-ordination number occurs when chloride ions are substituted for water ligands (usually six to four).

  • Substitution occurs stepwise and the stability of the complex ion that would be formed from another substitution determines whether a further substitution will happen (this is why only partial substitution sometimes occurs, for example with copper and ammonia).