Explain transition-element catalysis and complex formation
Relate variable oxidation states and accessible vacant d orbitals to catalytic behaviour and coordinate bonding.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 28. Use the first-row transition elements titanium to copper and the species explicitly named in the syllabus. Where redox potentials or stability constants are needed, use the values supplied in the question. Colour descriptions can depend on conditions and concentration. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, ionic equations, equilibrium expressions and the AS redox and bonding models.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What can an accessible vacant d orbital accept from a ligand?
What you will learn
- Explain catalytic behaviour using more than one stable oxidation state
- Explain how accessible vacant d orbitals can accept ligand lone pairs
- Connect these features to catalyst and complex formation without claiming they explain every catalyst
Variable oxidation states support catalytic cycles
A transition element can participate in successive steps at different oxidation states, then return to its original state. The ability to access more than one stable oxidation state can help a metal ion take part in a catalytic cycle.
Some transition-metal catalysts also have vacant d orbitals that are energetically accessible. These can accept lone pairs from reactants or ligands, forming temporary dative bonds that help provide an alternative pathway.
Vacant d orbitals accept a ligand lone pair
A complex forms when a central metal atom or ion is surrounded by ligands. A ligand donates a lone pair into an available orbital on the central species, making a dative covalent bond. In this topic, accessible vacant d orbitals are used to explain complex-ion formation.
The syllabus explanation is specific: variable oxidation states and accessible vacant d orbitals that can form dative bonds help explain transition-element catalysis; vacant accessible d orbitals help explain complex formation. Do not claim that every catalyst or every complex follows only one mechanism.
Pause and explain
How can variable oxidation states help in a catalytic cycle?
Worked example
Explain two features that can help a transition-metal ion catalyse a reaction.
Show the worked solution
- Identify the ability to use more than one stable oxidation state.
- Identify accessible vacant d orbitals.
- Explain how the latter can interact with a reactant lone pair.
Answer A transition-metal ion may change oxidation state during one step and return to its original state later. Its accessible vacant d orbitals can also accept ligand lone pairs and form temporary dative bonds, helping provide an alternative route.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A metal ion changes from M²⁺ to M³⁺ in one step and back to M²⁺ in a later step. Explain why this can be consistent with catalytic behaviour.
Give me a hint
Track the catalyst through both steps and compare its initial and final forms.
Compare my reasoning
- M²⁺ participates in the first step.
- M³⁺ is produced temporarily.
- The later step regenerates M²⁺.
The metal ion is changed during the cycle but is regenerated as M²⁺, so it is not consumed overall and can facilitate the reaction pathway.
Look for these in your work
- Temporary oxidation-state change identified
- Regeneration shown
- Catalytic role explained
Describe how a vacant, energetically accessible d orbital can contribute to complex formation.
Give me a hint
Follow the electron pair from the ligand to the central metal ion.
Compare my reasoning
- A ligand contains a lone pair.
- The metal ion has a vacant accessible orbital.
- Donation of the pair forms a dative covalent bond.
A ligand donates a lone pair into a vacant, accessible d orbital on the central metal ion, forming a dative covalent bond and contributing to the complex.
Look for these in your work
- Ligand donor identified
- Vacant orbital identified
- Bond formation described
A response says, ‘Transition metals are catalysts because their compounds are coloured.’ Replace this with a syllabus-level explanation.
Give me a hint
Use oxidation-state changes and accessible vacant orbitals, not the observed colour.
Compare my reasoning
- Mention more than one stable oxidation state.
- Explain that the metal can be used and regenerated in successive steps.
- Mention accessible vacant d orbitals forming dative bonds where relevant.
Their ability to access more than one stable oxidation state can allow a metal to be used in one catalytic step and regenerated later. Accessible vacant d orbitals can also accept ligand lone pairs and form dative bonds. Colour is not the explanation for catalytic behaviour.
Look for these in your work
- Relevant oxidation states used
- Regeneration stated
- Orbital interaction included
Common mistakes
- Saying the catalyst is permanently consumed
- Attributing catalysis only to colour or complex formation
- Describing a dative bond as donation of a proton
- Claiming all transition-metal catalysts use an identical cycle
What can you explain now?
What can an accessible vacant d orbital accept from a ligand?
Compare with the explanation
A lone pair of electrons
The ligand donates both bonding electrons into the available orbital.
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Make it stick
How do oxidation-state flexibility and accessible vacant d orbitals support transition-element catalysis?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.