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What makes the transition metals distinctive, and how do their complexes and colours arise?

The characteristic properties of transition metals, variable oxidation states, complex-ion formation and shapes, coloured ions and the d-d transition, ligand substitution, and catalysis.

An Eduqas A-Level Chemistry PI2.2 answer on the characteristic properties of transition metals, variable oxidation states, complex ions and their shapes, the origin of colour, ligand substitution and catalysis.

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  1. What this topic is asking
  2. Characteristic properties
  3. Variable oxidation states
  4. Complex ions and their shapes
  5. The origin of colour
  6. Ligand substitution
  7. Catalysis
  8. Examples in context
  9. Try this

What this topic is asking

Eduqas topic PI2.2 covers the distinctive chemistry of the d-block transition metals: their characteristic properties, variable oxidation states, the formation and shapes of complex ions, the origin of their colours, ligand substitution reactions, and their role as catalysts. It is a rich, exam-favourite topic that draws on bonding and electrode-potential ideas.

Characteristic properties

Variable oxidation states

Because the 4s4\text{s} and 3d3\text{d} sub-shells are close in energy, transition metals lose different numbers of electrons to give several oxidation states (iron is commonly +2+2 and +3+3; manganese ranges up to +7+7 in MnO4−\text{MnO}_4^-). This underlies their redox chemistry and catalysis.

Complex ions and their shapes

The origin of colour

The size of the gap (and so the colour) depends on the metal ion, its oxidation state, and the identity and arrangement of the ligands, which is why ligand substitution changes the colour.

Ligand substitution

In a ligand substitution reaction, one ligand replaces another around the metal ion. Adding concentrated ammonia or hydrochloric acid to the copper(II) aqua ion replaces water with ammonia (deep blue) or chloride (yellow, and tetrahedral); the colour and sometimes the shape change. Multidentate ligands can displace monodentate ones in a favourable, entropy-driven exchange.

Catalysis

Transition metals and their ions are widely used as catalysts because they offer accessible variable oxidation states (for homogeneous catalysis) or surfaces that adsorb reactants (for heterogeneous catalysis). Iron in the Haber process and vanadium(V) oxide in the Contact process are standard examples.

Examples in context

Example 1. Haemoglobin. The iron(II) centre in haemoglobin is a complex ion that reversibly binds oxygen as a ligand; carbon monoxide is toxic because it binds more strongly, a direct biological application of ligand substitution.

Example 2. Colour in gemstones. Traces of transition-metal ions give rubies (chromium) and sapphires (iron and titanium) their colours through d-d transitions, the same mechanism that colours laboratory complexes.

Try this

Q1. State why Zn2+\text{Zn}^{2+} is colourless whereas Cu2+\text{Cu}^{2+} is coloured. [2 marks]

  • Cue. Zn2+\text{Zn}^{2+} has a full 3d103\text{d}^{10} sub-shell, so no d-d transition is possible; Cu2+\text{Cu}^{2+} has a partially filled d\text{d} sub-shell, so an electron can be promoted and visible light absorbed.

Q2. Write the formula and state the shape of the complex ion formed when copper(II) ions react with excess chloride ions. [2 marks]

  • Cue. [CuCl4]2−[\text{CuCl}_4]^{2-}, which is tetrahedral.

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20194 marks(a) State three characteristic properties of transition metals. (b) Explain why aqueous copper(II) ions are blue.
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(a) Any three (1 each, max 3): they form ions with variable oxidation states; they form coloured ions/compounds; they (or their compounds) act as catalysts; they form complex ions. (Also acceptable: high melting points and densities.)

(b) In the complex ion the d\text{d} orbitals are split into two energy levels by the ligands; an electron absorbs visible light to be promoted from the lower to the higher level (a d-d transition) (1). The colour seen (blue) is the complementary colour of the wavelengths absorbed (1).

Eduqas 20214 marksWhen concentrated hydrochloric acid is added to aqueous copper(II) sulfate, the blue solution turns yellow-green. (a) Name the type of reaction. (b) Write an equation and explain the colour change in terms of the ligands and complex shape.
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(a) Ligand substitution (1).

(b) [Cu(H2O)6]2++4Cl−⇌[CuCl4]2−+6H2O[\text{Cu(H}_2\text{O})_6]^{2+} + 4\text{Cl}^- \rightleftharpoons [\text{CuCl}_4]^{2-} + 6\text{H}_2\text{O} (1). The six water ligands (octahedral, blue) are replaced by four chloride ligands (tetrahedral, yellow) (1). The different ligand and shape change the d-orbital splitting, so different wavelengths are absorbed and the colour changes (1).

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