Quick Notes - A2 Inorganic Chemistry

Transition Metals

Transition Metal Properties

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
- act as catalysts

Transition Metals and Colour

The d-orbitals in a transition metal ion can be split into high energy and low energy orbitals. This creates an energy gap 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 colours.
- If the energy required to excite an electron can be found in white light, then specific wavelengths of light are absorbed by the transition metal ion and removed from the white light.
- The complimentary colour of the absorbed wavelengths is now observed, and the transition metal ion 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.
- Some d-orbitals are repelled more by approaching ligands than others, making them higher energy (and the other d-orbitals lower energy).
- The d-orbitals are split into two groups: high energy and low energy, with a specific energy gap between them.

Colorimetry

Colorimetry is a technique used to find the concentration of a solution based on the intensity of light (of a specific wavelength) it absorbs or scatters.
- Coloured solutions absorb specific wavelengths of visible light. The more concentrated a solution is, the greater the intensity of light it absorbs.
The amount of light that passes through a sample is detected, giving an indication as to the amount of light absorbed by the sample.
- A calibration curve is made using the absorbances of solutions of known concentrations.
- The absorbance of the unknown concentration is compared to the calibration curve, and the concentration is 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 ratio of the oxidised species to the reduced species, the moles of the other species can be found and the concentration calculated.

Transition Metals as Catalysts

Transition metals often make good catalysts.
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, which can then gain or lose electrons to reform the original ion.
- In a reaction, this means a transition metal can provide an alternative route, allowing a species to be oxidised and another species to be reduced.

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
- reactants are absorbed onto the catalyst
- the reaction starts and an intermediate is formed
- the reaction completes and products are 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, as they have variable oxidation states and can easily oxidise and reduce other aqueous ions.

A positively charged 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 positively charged 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 and an octahedral shape.
- Co-ordination number refers to how many co-ordinate bonds there are in total 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 Ions - Precipitation Reactions

Metal aqua complex ions can behave as (very) weak acids in solution.
- High polarisation 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 in the aqua complex 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 greater the positive charge of the metal ion, the more acidic the complex ion.

Ligand Substitution

Ligands in a metal complex ion can be substituted for another type of ligand.
- 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).