Variable oxidation states, ligand substitution, colour, and catalysis - key concepts, worked examples, and exam tips for H2 Chemistry.
Q: What does H2 Chemistry Notes: Topic 13 - Transition Elements cover? A: Explore electronic configurations, variable oxidation states, ligand behaviour, colour, and catalysis for the transition elements extension topic.
Transition-metal chemistry blends electronic structure with observable properties such as coloured ions and catalytic behaviour.
This note highlights the examinable concepts for the 2026 syllabus.
Status: SEAB's current H2 Chemistry (9476) syllabus PDF is labelled for 2026, and the current Chemistry Data Booklet is labelled 8873/9476/9813 for use from 2026 in non-practical papers.
The core idea is simple: Transition elements are tested through colour, complexes, oxidation states, and catalysis.
Use it as a working check: Link every observation to structure: metal ion, oxidation state, ligand, coordination number, and geometry.
Then go one layer deeper: Example: if a solution changes from pale green Fe(II) to yellow-brown Fe(III), say oxidation state changed from +2 to +3, then connect that to the redox reagent used.
Quick revision box
What this topic tests: Transition-metal trends, complexes, colour changes, redox behaviour, catalysis.
Top mistakes to avoid: Vague ligand-field explanations; missing oxidation-state transitions; weak link between observations and structure.
20-minute sprint plan: 5 min complex-ion colour map; 10 min redox/ligand exercises; 5 min catalysis + observation explanation.
Route map: choose the transition-element idea first
If the question asks about...
Start with...
Then connect to...
Trap to avoid
Whether an element is a transition element
The d-electron count in the atom or stable ion
Partially filled d subshell or a clear exclusion
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Do not quote colour changes without naming the oxidation-state change.
Complex formula or shape
Ligand charge and denticity
Coordination number, geometry, and overall charge
Do not omit coordinated water or ammonia molecules from the formula.
Colour of a complex
Metal ion, ligand, oxidation state, and geometry
d-orbital splitting and complementary colour
Do not use d-d transition language for d0 or d10 ions.
Ligand substitution observation
The ligand added and whether it is in excess
Precipitate, soluble complex, and colour sequence
Do not skip the dropwise intermediate when ammonia is added to copper(II).
Catalysis explanation
Homogeneous or heterogeneous pathway
Oxidation-state cycle or surface adsorption
Do not stop at "variable oxidation states" without showing the cycle.
Use this map before writing an explanation. Transition-element marks usually come from linking the visible observation to the metal ion, ligand environment, and electron-level reason.
1 Defining Transition Elements
Transition elements have partially filled d subshells in atoms or stable ions. Scandium ScX3+ and zinc ZnX2+ lack d electrons in common oxidation states, so they are typically excluded.
When deciding whether an element counts as a transition element, test the atom and at least one common stable ion. Remove 4s electrons before 3d electrons when forming the ion.
Species to test
Electron count move
Classification decision
Common trap
Sc atom
[Ar]3d14s2 has one 3d electron.
The atom alone looks promising, but the common stable ScX3+ ion is [Ar].
Stopping at the atom and forgetting the stable ion.
ScX3+ ion
Remove two 4s electrons and one 3d electron.
No partially filled d
Zn atom
[Ar]3d104s2 has a full 3d subshell.
The atom does not have a partially filled d
ZnX2+ ion
Remove two 4s electrons.
[Ar]3d10
FeX2+ ion
Remove two 4s electrons from Fe.
[Ar]3d6
Worked check: copper has [Ar]3d104s1 as an atom, but CuX2+ has [Ar]3d9. Since the common ion has a partially filled d subshell, copper is treated as a transition element.
Misconception check: "d-block" and "transition element" are not identical labels. The transition-element test is about a partially filled d subshell in the atom or a stable ion.
2 Variable Oxidation States
Transition metals exhibit multiple oxidation states due to similar energy of 3d and 4s electrons. Highlight trends:
Early transition metals favour higher oxidation states.
Late transition metals stabilise lower states.
Provide examples:
Metal
Oxidation states
Signature species
Manganese
+2, +4, +7
MnX2+, MnOX2, MnOX4X−
Iron
+2, +3
FeX2+, FeX3+
Copper
+1, +2
CuX+, CuX2+
Oxidation-state accounting checkpoint
When a transition-element question gives a formula, calculate the oxidation state before explaining colour, redox, or catalysis. This stops you from guessing the metal charge from memory.
Species in the formula
Accounting move
Worked result
Common trap
MnOX4X−
Let Mn be x; four oxide ions contribute −8.
x−8=−1, so Mn is +7.
Calling it MnX2+ because manganese often forms +2.
[Fe(CN)X6]X4−
Each CNX−
[Cu(NHX3)X4(HX2O)X2]X2+
VX2OX5
Five oxide ions contribute −10
Worked check: in [Fe(CN)X6]X3−, let Fe be x. The six cyanide ligands contribute −6, so x−6=−3, giving Fe as +3. That oxidation-state change from +2 to +3 matters when you compare colour, magnetic behaviour, or redox role.
Misconception check: ligand charge and complex charge are different pieces of information. Add all ligand charges first, then use the overall complex charge to solve for the metal.
3 Ligands and Coordination
Ligand: species donating lone pair to metal to form coordinate bond.
Monodentate: binds via one donor atom (HX2O, NHX3, ClX−).
Bidentate/polydentate: e.g. en (ethane-1,2-diamine), EDTAX4−.
Coordination number: number of coordinate bonds to metal.
Common geometries:
Coordination number
Geometry
Example
4
Tetrahedral
[CuClX4]X2−
4
Square planar
[Pt(NHX3)X2ClX2]
6
Octahedral
[Fe(HX2O)X6]X3+
Complex formula checkpoint
When a question asks for a complex formula or oxidation state, separate ligand charge, coordination number, and overall charge before writing the final species.
Prompt clue
First check
Example setup
Aqua or ammine complex
Ligand is neutral.
Six HX2O around FeX3+ gives [Fe(HX2O)X6]X3+.
Chloro complex
Each ClX− contributes −1.
Four ClX−
Bidentate ligand
One ligand forms two coordinate bonds.
Three neutral en ligands give coordination number 6, not 3.
Overall complex charge given
Let metal oxidation state be x.
Add metal charge and ligand charges, then equate to the complex charge.
Misconception check: the subscript after a ligand is not always the coordination number. Multiply ligand count by denticity when the ligand is bidentate or polydentate.
Denticity counting checkpoint
When a complex contains chelating ligands, count donor bonds, not just ligand formula units. This is the step that links the formula to the coordination number and shape.
Complex clue
Donor-bond count
Coordination-number conclusion
Trap to avoid
Six monodentate water ligands, such as [Fe(HX2O)X6]X3+
6×1=6 donor bonds
Coordination number 6, usually octahedral.
This is the only case where ligand count and coordination number match directly.
Three neutral en ligands, such as [Co(en)X3]X3+
3×2=6
Two oxalate ligands plus two water ligands
2×2+2×1=6 donor bonds
Coordination number 6 if each oxalate is bidentate.
Counting only four ligands and choosing a 4-coordinate shape.
One EDTAX4− ligand
Usually six donor bonds from one polydentate ligand
One ligand can still give coordination number 6.
Treating "one ligand" as coordination number 1.
Worked check: in [Co(en)X3]X3+, each en ligand is neutral and bidentate. The three en ligands supply six donor bonds, so the coordination number is 6. Since en is neutral and the overall complex charge is +3, cobalt is in the +3 oxidation state.
Misconception check: ligand count answers "how many ligand formula units are written"; coordination number answers "how many coordinate bonds reach the metal". They match only when every ligand is monodentate.
3.1 Ligand 2D References
Ammonia: monodentate ligand (donor atom N).
Ethane-1,2-diamine (en): bidentate ligand reference.
Oxalate: bidentate anionic ligand used as a chelating reference.
4 Colour and Crystal Field Theory (CFT)
Ligands split d orbitals into different energy levels. Absorption of specific wavelengths promotes d-d transitions; observed colour corresponds to complementary wavelength.
Factors affecting colour:
Ligand field strength: Different ligands split the d orbitals by different amounts (for example, CNX− generally produces a larger splitting than ClX−).
Oxidation state: Higher state increases splitting, shifting absorption.
Geometry: Octahedral vs tetrahedral splitting patterns differ.
Colour explanation checkpoint
When explaining colour, move from electronic structure to the observed solution. This prevents answers from stopping at "d orbitals split" without linking the observation to the complex.
Question clue
First check
Explanation step to include
Common trap
A complex is coloured
Check the metal ion has a partially filled d subshell.
Ligands split the d orbitals; absorbed light promotes a d electron; the complementary colour is observed.
Forgetting to mention the absorbed colour and observed colour are complementary.
A complex is colourless
Check whether the ion is d0 or d10.
No suitable d-d transition is possible, so visible light is not selectively absorbed.
Saying it is colourless because there are no ligands.
Ligand changes but metal oxidation state is the same
Compare ligand field strength and geometry.
A different splitting energy means a different wavelength is absorbed, so the observed colour changes.
Claiming the colour changes only because the formula looks different.
Oxidation state changes
Name the new metal ion or oxidation state first.
A different d-electron count and metal charge change the splitting and possible transitions.
Quoting a colour change without linking it to electron arrangement.
Common trap: do not use d-d transition language for every d-block ion. The colour explanation depends on the complex having an available d electron transition.
Explain why [Cu(HX2O)X6]X2+ is blue but [CuClX4]X2− is yellow-green (weaker ligand reduces splitting, absorbing lower energy).
5 Ligand Substitution
Ligand exchange affects colour and stability. Example:
[Cu(HX2O)X6]X2++4NHX3[Cu(NHX3)X4(HX2O)X2]X2++4HX2O
Observation: pale blue solution turns deep blue. Provide stepwise explanation-ammonia acts as ligand and base (careful with precipitation intermediate Cu(OH)X2 when adding NHX3 dropwise).
Ligand substitution sequence checkpoint
When aqueous ammonia or hydroxide is added to a transition-metal ion, write the observation sequence before writing the final complex. This prevents you from jumping straight to the excess-ligand product and missing the intermediate precipitate.
Stage in the test
What to write first
Chemical reason
Trap to avoid
A few drops of aqueous ammonia are added to CuX2+
Pale blue precipitate forms.
Ammonia acts as a base and produces OHX−, forming Cu(OH)X2.
Calling the first observation a deep blue solution.
Excess aqueous ammonia is added
Precipitate dissolves to give a deep blue solution.
Ammonia ligands replace water ligands to form an ammine complex.
Forgetting the word "excess" before naming the soluble complex.
Aqueous sodium hydroxide is added instead
Pale blue precipitate remains insoluble in excess.
Hydroxide forms the precipitate but does not form the same deep blue ammine complex.
Treating all bases as if they behaved like excess ammonia.
Worked check: if the question says "add aqueous ammonia dropwise, then in excess", your answer needs two observations: pale blue precipitate first, then deep blue solution in excess. The final deep blue colour alone is incomplete because it skips the dropwise stage.
Misconception check: a reagent can behave differently at different stages. In this test, ammonia first supplies hydroxide ions as a base, then acts as a ligand when present in excess.
Ligand substitution driver: excess ammonia.
6 Catalytic Behaviour
Transition metals and complexes act as catalysts because:
They provide variable oxidation states for electron transfer (e.g. VX2OX5 in the Contact process: VX5+/VX4+ cycle).
They adsorb reactants on surfaces (heterogeneous catalysis).
Complex ions facilitate alternative pathways (homogeneous catalysis).
Describe mechanism for FeX2+ catalysing iodide-thiosulfate reaction or CoX2+ catalysing decomposition of hydrogen peroxide.
Catalysis mechanism checkpoint
Before writing a catalysis explanation, decide whether the catalyst is helping by changing oxidation state, by providing a surface, or by forming a temporary complex. The scoring point is the lower-energy pathway, not just the word "catalyst".
Catalyst clue
Mechanism to describe
Answer move
Transition-metal ion in solution
Homogeneous redox cycle
Show the metal ion being oxidised in one step and reduced back in another, so it is regenerated.
Solid transition metal or metal oxide
Heterogeneous surface catalysis
State that reactants adsorb on the surface, bonds are weakened or aligned, products form, then desorb.
Complex ion or ligand-rich solution
Temporary complex formation
Explain that coordination brings reactants together or changes electron density, then the catalyst is released.
Question gives two half-reactions
Oxidation-state shuttle
Match one half-reaction to metal oxidation and the other to metal reduction.
Worked check: in a redox-cycle answer, do not write only "the catalyst has variable oxidation states". Name the two oxidation states and show that the catalyst returns to its original state by the end of the cycle.
Misconception check: a catalyst is not consumed overall, but it can be changed during an intermediate step. The key is regeneration by the final step.
7 Worked Example
Question:
You are given an unknown solution that contains either FeX2+ or FeX3+. Describe what you observe when you add (i) aqueous NaOH and (ii) aqueous NHX3, and explain how you can distinguish the ions.
Answer:
The SEAB Chemistry Data Booklet’s qualitative analysis notes give these observations:
With NaOH(aq):
FeX2+: green precipitate forms, turning brown on contact with air; insoluble in excess.
FeX3+: red-brown precipitate forms; insoluble in excess.
With NHX3(aq):
FeX2+
So, a fresh green precipitate that browns on standing indicates FeX2+, while an immediate red-brown precipitate indicates FeX3+.
8 Qualitative Analysis Integration
These are high-frequency observations directly listed in the SEAB Chemistry Data Booklet:
CuX2+: pale blue precipitate with NaOH(aq) (insoluble in excess); with NHX3(aq), the blue precipitate dissolves in excess to give a deep blue solution.
CrX3+: grey-green precipitate with NaOH(aq) (soluble in excess, giving a dark green solution); grey-green precipitate with NHX3(aq)
FeX2+ vs FeX3+: green precipitate turning brown (FeX2+
Remember to connect observations to oxidation states and ligand changes; colour descriptions align with the SEAB Chemistry Data Booklet tables.
9 Common Misconceptions
Assuming all d-block elements form coloured ions-d0 or d10 complexes (e.g. ScX3+, ZnX2+) are colourless.
Overlooking tetrahedral vs square-planar distinction in 4-coordinate complexes.
Forgetting to include water molecules in coordination sphere when writing formulae.
Confusing oxidation numbers when counting ligands (neutral ligands like HX2O contribute 0).
10 Quick Drills
Write the stepwise equations for redox cycling of MnOX4X− in acidic, neutral, and alkaline media, including colours.
Predict the magnetic behaviour (paramagnetic/diamagnetic) of [Fe(CN)X6]X4− and [Fe(HX2O)X6]X2+; explain using electron configuration and ligand field strength.
Show how [Cr(HX2O)X6]X3+
Common exam mistakes
Mistake: Claiming Sc and Zn are transition elements - ScX3+ has an empty d subshell and ZnX2+ has a completely filled d subshell, so neither qualifies as a transition element by the standard definition.
Mistake: Writing the wrong electronic configuration for Cr and Cu - Cr is [Ar]3d54s1 and Cu is [Ar]3d104s1
Mistake: Stating that colourless complexes have d-d transitions - d0 or d10 complexes (e.g. ScX3+
Mistake: Describing ligand substitution observations without the correct intermediate - when aqueous ammonia is added dropwise to CuX2+, a pale blue precipitate forms first (Cu(OH)X2
Mistake: Omitting water molecules from the coordination sphere in formulae - the aqua complex [Cu(HX2O)X6]X2+
Mistake: Giving a vague catalysis explanation - "transition metals act as catalysts because they have variable oxidation states" must be supported by showing the actual oxidation-state cycle (e.g. VX5+VX4+VX5+
Frequently asked questions
Why are transition metal ions coloured? In a transition metal complex, ligands split the d orbitals into two energy levels. When light is absorbed to promote an electron between these levels (a d-d transition), the complementary colour is observed. Ions with no d electrons (d0) or a full d subshell (d10) cannot undergo this transition and appear colourless.
How do I predict the geometry of a complex ion? Coordination number 6 gives an octahedral geometry (e.g. [Fe(HX2O)X6]X3+). Coordination number 4 is most commonly tetrahedral (e.g. [CuClX4]X2−), but square planar geometry also occurs (e.g. [Pt(NHX3)X2ClX2]). The SEAB syllabus expects you to state and draw these geometries correctly.
Why do transition metals act as catalysts? Transition metals can exist in multiple oxidation states because the 3d and 4s orbitals are close in energy. This allows them to accept and donate electrons in catalytic cycles (homogeneous catalysis via variable oxidation states) or to adsorb and activate reactant molecules on their surfaces (heterogeneous catalysis).
What is the difference between monodentate and bidentate ligands? A monodentate ligand has one donor atom and forms one coordinate bond to the metal (e.g. NHX3, HX2O, ClX−). A bidentate ligand has two donor atoms and forms two coordinate bonds simultaneously (e.g. ethane-1,2-diamine, oxalate), creating more stable chelate complexes due to the chelate effect.
Struggling with Transition Elements? Our H2 Chemistry tuition programme covers this topic with structured practice, Paper 4 practical drills, and worked exam solutions.
