A-Level Chemistry AQA 7405

General properties of transition metals

Transition metal characteristics of elements Ti–Cu arise from an incomplete d sub-level in atoms or ions.

The characteristic properties include:

• complex formation
• formation of coloured ions
• variable oxidation state
• catalytic activity.

A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.

A complex is a central metal atom or ion surrounded by ligands.

Co-ordination number is number of co-ordinate bonds to the central metal atom or ion.

Substitution reactions

H₂O, NH₃ and Cl⁻ can act as monodentate ligands.

The ligands NH₃ and H₂O are similar in size and are uncharged.

Exchange of the ligands NH₃ and H₂O occurs without change of co-ordination number (eg Co²⁺ and Cu²⁺).

Substitution may be incomplete (eg the formation of Cu(NH₃)₄(H₂O)2²⁺).

The Cl^– ligand is larger than the uncharged ligands NH₃ and H₂O

Exchange of the ligand H₂O by Cl^– can involve a change of co-ordination number (eg Co²⁺, Cu²⁺ and Fe³⁺).

Ligands can be bidentate (eg H₂NCH₂CH₂NH₂ and C₂O₄^2–).

Ligands can be multidentate (eg EDTA^4–).

Haem is an iron(II) complex with a multidentate ligand.

Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood.

Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin.

Bidentate and multidentate ligands replace monodentate ligands from complexes. This is called the chelate effect.

Students should be able to explain the chelate effect, in terms of the balance between the entropy and enthalpy change in these reactions.

Shapes of complex ions

Transition metal ions commonly form octahedral complexes with small ligands (eg H₂O and NH₃).

Octahedral complexes can display cis–trans isomerism (a special case of E–Z isomerism) with monodentate ligands and optical isomerism with bidentate ligands.

Transition metal ions commonly form tetrahedral complexes with larger ligands (eg Cl^–).

Square planar complexes are also formed and can display cis–trans isomerism.

Cisplatin is the cis isomer.

Ag+ forms the linear complex Ag(NH₃)₂⁺ as used in Tollens’ reagent.

Formation of coloured ions

Transition metal ions can be identified by their colour.

Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected.

d electrons move from the ground state to an excited state when light is absorbed.

The energy difference between the ground state and the excited state of the d electrons is given by:

$∆E = hν = \dfrac{hc}{λ}$

Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour.

The absorption of visible light is used in spectroscopy.

A simple colorimeter can be used to determine the concentration of coloured ions in solution.

Variable oxidation states

Transition elements show variable oxidation states.

Vanadium species in oxidation states IV, III and II are formed by the reduction of vanadate(V) ions by zinc in acidic solution.

The redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by pH and by the ligand.

The reduction of Ag(NH₃)₂⁺ (Tollens’ reagent) to metallic silver is used to distinguish between aldehydes and ketones.

The redox titrations of Fe²⁺ and C₂O₄^2– with MnO₄^–

Students should be able to perform calculations for these titrations and similar redox reactions.

Catalysts

Transition metals and their compounds can act as heterogeneous and homogeneous catalysts.

A heterogeneous catalyst is in a different phase from the reactants and the reaction occurs at active sites on the surface.

The use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise the cost.

V₂O₅ acts as a heterogeneous catalyst in the Contact process.

Fe is used as a heterogeneous catalyst in the Haber process.

Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency; this has a cost implication.

A homogeneous catalyst is in the same phase as the reactants.

When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species.

Students should be able to:

• explain the importance of variable oxidation states in catalysis
• explain, with the aid of equations, how V₂O₅ acts as a catalyst in the Contact process
• explain, with the aid of equations, how Fe²⁺ ions catalyse the reaction between I^– and S₂O₈^2–
• explain, with the aid of equations, how Mn²⁺ ions autocatalyse the reaction between C₂O₄^2– and MnO₄^–