Q: What does H2 Chemistry Notes: EXTENSION TOPICS, 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.
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
\(\ce{Mn^{2+}}\), \(\ce{MnO2}\), \(\ce{MnO4^-}\)
Iron
+2, +3
\(\ce{Fe^{2+}}\), \(\ce{Fe^{3+}}\)
Copper
+1, +2
\(\ce{Cu^+}\), \(\ce{Cu^{2+}}\)
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
\(\ce{[CuCl4]^{2-}}\)
4
Square planar
\(\ce{[Pt(NH3)2Cl2]}\)
6
Octahedral
\(\ce{[Fe(H2O)6]^{3+}}\)
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: Spectrochemical series IX−<BrX−<ClX−<HX2O<NHX3<en<CNX−.
Oxidation state: Higher state increases splitting, shifting absorption.
Geometry: Octahedral vs tetrahedral splitting patterns differ.
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).
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.
7 Worked Example
Question:
Explain the colour changes observed when aqueous FeX2+ is oxidised to FeX3+ and then treated with potassium thiocyanate (KSCN).
Answer:
Oxidation:FeX2+ (pale green) oxidised by HX2OX2 or MnOX4X− yields FeX3+ (yellow-brown). Colour deepens due to higher charge density causing greater ligand field splitting.
Complex formation:FeX3++SCNX−[Fe(SCN)]X2+
Mention charge transfer transitions contributing to intense colour.
8 Qualitative Analysis Integration
Precipitation:FeX2+ with OHX− forms green Fe(OH)X2 that oxidises to brown Fe(OH)X3.
Ligand substitution:[Co(HX2O)X6]X2+
Redox:CrX2OX7X2−
Remember to connect observations to oxidation states and ligand changes.
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+
Transition-element chemistry demands linking electronic explanations to laboratory evidence. Practise writing full ionic equations, annotate colour changes precisely, and leverage the wider resource bank at https://eclatinstitute.sg/blog/h2-chemistry-notes.
H2 Chemistry Notes: EXTENSION TOPICS, Topic 13 - Transition Elements
. The thiocyanate ligand creates an intense blood-red complex because
π
-acceptor ligand increases crystal field splitting, absorbing blue-green light.
pink; with concentrated
HCl
forms
[CoClX4]X2−
blue.
orange reduced to
CrX3+
green; further reduction to
CrX2+
blue.
reacts with aqueous ammonia in stages, including colour observations.
