IB Chemistry Transition Metals & Complex Ions (HL): FAQ

Transition metals and complex ions sit entirely within Structure 3.1 as additional higher level (AHL) content — SL students aren't examined on it at all. Expect short-answer or extended-response questions in Paper 2 asking you to explain colour, oxidation states or ligand behaviour, plus occasional data-based questions in Paper 3.

The IB defines a transition element as one whose atom, or a stable ion it forms, has an incomplete d sub-shell. That's why scandium and zinc are technically d-block elements but not transition metals — Sc³⁺ has no d electrons at all, and Zn²⁺ has a full 3d¹⁰ shell.

Colour comes from d-d electron transitions. Ligands bonded to the metal split the five degenerate d orbitals into two energy levels — crystal field splitting — and an electron absorbs a specific wavelength of visible light jumping between them. What you see is the complementary colour of the light absorbed, not the light itself.

A ligand is a molecule or ion with at least one lone pair that forms a dative (coordinate) bond to a central metal ion. Coordination number counts the total number of these dative bonds, not the number of ligand molecules — six NH3 ligands and three bidentate ethylenediamine ligands both give coordination number 6.

Transition metals show variable oxidation states because their 3d and 4s electrons are so close in energy that similar amounts of energy are needed to remove different numbers of them. Iron commonly forms both Fe²⁺ and Fe³⁺, and neither is dramatically more stable, so both persist in solution and in compounds.

The mistake I see every single year: students write chromium as [Ar]3d⁴4s² and copper as [Ar]3d⁹4s², instead of the correct anomalous configurations [Ar]3d⁵4s¹ and [Ar]3d¹⁰4s¹. Both arise because a half-filled or fully-filled d sub-shell is more stable — examiners award the mark for stating why, not just for the configuration itself.

For cis-trans isomerism, draw the square planar or octahedral complex and show two identical ligands either adjacent (cis) or opposite (trans) — cisplatin, [Pt(NH3)2Cl2], is the classic IB example. For optical isomerism, draw both non-superimposable mirror images of an octahedral complex with three bidentate ligands, like [Ni(en)3]2+, and label them enantiomers.

The IB-approved explanation has two parts. Variable oxidation states let a transition metal provide an alternative reaction pathway with lower activation energy — homogeneous catalysis, like Fe²⁺/Fe³⁺ in the iodine-persulfate reaction. Partially filled d orbitals let reactant molecules adsorb onto the metal surface, weakening bonds — heterogeneous catalysis, like iron in the Haber process.

HL only. Electron configurations of d-block elements, complex ion formation, colour, catalytic behaviour and isomerism are all additional higher level (AHL) material in Structure 3.1 of the current DP Chemistry guide. If your child is doing SL Chemistry, they will not be examined on ligands or crystal field splitting.

Most students manage naming complexes and calculating oxidation states without much trouble. Crystal field theory and colour trip more people up, because you need to link three ideas — ligand field strength, electron promotion and wavelength absorbed — into one coherent answer. It rewards clear, structured writing more than raw memorisation.

There's no fixed weighting the IB publishes for this specific subtopic, but transition metals and complex ions typically generate one extended-response question worth roughly 6-10 marks in Paper 2, plus occasional data-based questions in Paper 3 involving absorption spectra or reaction-rate data. It's a reliably recurring topic across recent exam sessions.

Three question types recur across past papers: explain why a named complex is coloured using crystal field theory, deduce the oxidation state and coordination number of a metal from a given formula, and draw or name the cis-trans or optical isomers of a specified complex ion. Practising all three under timed conditions covers most of what actually gets asked.

The students who do well on this topic aren't the ones who've memorised definitions — they're the ones who can draw a complex ion, name it correctly and explain its colour in one connected answer. Short, regular practice with past-paper style questions beats re-reading notes the night before a mock, every time.