What is a transition metal?

Definition: a transition metal is a d-block element that forms at least one stable ion with an incomplete d sub-level.

That precise wording matters, because it excludes two d-block elements:

• Scandium — its only ion is Sc³⁺, which is [Ar] (an empty d sub-level).
• Zinc — its only ion is Zn²⁺, which is [Ar] 3d¹⁰ (a full d sub-level).

Neither has an incomplete d sub-level in its ion, so neither is a transition metal — and, tellingly, neither shows the characteristic properties. All four properties flow from that partly filled d sub-level:

• complex formation · coloured ions · variable oxidation state · catalytic activity

Remember the ion, not the atom. Copper is a transition metal because Cu²⁺ is [Ar] 3d⁹ — even though the copper atom is [Ar] 4s¹3d¹⁰. And do not forget: 4s electrons are removed before 3d when the ion forms.

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Complex ions, ligands and shape

A ligand is a molecule or ion that donates a lone pair of electrons to a metal ion to form a co-ordinate (dative) bond. A complex is a central metal ion surrounded by ligands. The co-ordination number is the number of co-ordinate bonds to the central ion.

• Monodentate — one lone pair each: H₂O, NH₃, Cl⁻. H₂O and NH₃ are similar in size and uncharged, so they exchange without changing the co-ordination number.
• Bidentate — two donor atoms each: H₂NCH₂CH₂NH₂ (ethane-1,2-diamine) and the ethanedioate ion, C₂O₄²⁻.
• Multidentate — EDTA⁴⁻ forms six co-ordinate bonds all by itself. Haem is an Fe(II) complex with a multidentate ligand.

Shape depends on ligand size: small ligands (H₂O, NH₃) usually give six-co-ordinate octahedral complexes; the larger Cl⁻ ligand usually gives four-co-ordinate tetrahedral ones. [Ag(NH₃)₂]⁺ (Tollens’ reagent) is linear. Square planar complexes also occur — cisplatin is the cis isomer of a square planar platinum complex, used to treat cancer.

Why carbon monoxide is toxic: haemoglobin transports O₂ by forming a co-ordinate bond to Fe(II). CO binds to the same site far more strongly and is not readily released, so the haemoglobin can no longer carry oxygen.

Your turn

Your turn

Sort each complex ion by its shape

Tap a complex, then tap its shape. Think about the size of the ligand.

Ligand substitution and the chelate effect

Ligands can be swapped. With excess ammonia, copper(II) does a partial substitution — only four of the six waters are replaced:

With concentrated HCl the bigger chloride ligand forces a change of co-ordination number:

The chelate effect: a bidentate or multidentate ligand will displace monodentate ligands, and the driving force is almost entirely entropy. For example, EDTA⁴⁻ replaces six water molecules: 2 particles become 7. Because the ΔH of the reaction is close to zero (a similar number of similar co-ordinate bonds is broken and made), the large positive ΔS makes ΔG = ΔH − TΔS negative.

Exam phrasing: "there is an increase in the number of particles, so ΔS is positive; ΔH is approximately zero, so ΔG is negative and the substitution is feasible." That sentence is worth full marks.

Why transition metal ions are coloured

Ligands split the five d orbitals into two energy levels. When light passes through, a d electron absorbs a photon and jumps from the ground state to an excited state. The energy gap is:

The colour you see is the light that is not absorbed — the transmitted complement. Anything that changes ΔE changes the colour, so the colour depends on the:

• oxidation state of the metal · co-ordination number · identity of the ligand

A colorimeter uses this: absorbance is proportional to concentration, so you plot a calibration curve of absorbance against known concentrations and read off an unknown. (You select the filter of the colour that the solution absorbs most — the complementary colour to the one you see.)

Why Sc³⁺ and Zn²⁺ solutions are colourless: Sc³⁺ has no d electrons and Zn²⁺ has a full d sub-level, so in neither case can a d electron jump to a vacant higher d orbital. No absorption, no colour.

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Variable oxidation states and redox titrations

Transition metals show variable oxidation states because the 4s and 3d sub-levels are close in energy, so a variable number of electrons can be lost. Vanadium is the classic example: VO₂⁺ (+5, yellow) → VO²⁺ (+4, blue) → V³⁺ (+3, green) → V²⁺ (+2, violet), reduced step by step by zinc in acid.

Manganate(VII) titrations are self-indicating: MnO₄⁻ is intensely purple and Mn²⁺ is almost colourless, so the end point is the first permanent pale pink.

Acidify with dilute sulfuric acid — never hydrochloric (Cl⁻ would be oxidised to Cl₂, so too much MnO₄⁻ would be used) and never nitric (it is itself an oxidising agent).

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Heterogeneous and homogeneous catalysis

Heterogeneous — the catalyst is in a different phase from the reactants (usually a solid with gases). The reaction occurs at active sites on the surface: reactants adsorb, react, and then desorb. The catalyst is spread over a support medium to maximise surface area and minimise cost. It can be poisoned by impurities that block the active sites (e.g. sulfur poisoning the iron in the Haber process, or lead poisoning a catalytic converter) — which reduces efficiency and costs money.

Homogeneous — the catalyst is in the same phase as the reactants, and the reaction proceeds via an intermediate species.

• S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺ (charge: −2 + 4 = +2 = −4 + 6 ✓)
• 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ (charge: +6 − 2 = +4 = +4 ✓)

Each step is between oppositely charged ions, so both are far faster — and the Fe²⁺ is regenerated. This only works because iron has a variable oxidation state.

Autocatalysis: in the MnO₄⁻ / C₂O₄²⁻ titration, the Mn²⁺ product is itself the catalyst. That is why the first few drops decolourise slowly and then the reaction suddenly speeds up.

Match each catalyst to its process

Tap a catalyst on the left, then the process it catalyses on the right.

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Definitions, colour and the chelate effect

• The definition is about the ION, not the atom. "A transition metal forms at least one stable ion with an incomplete d sub-level." That is why Sc (Sc³⁺ = empty d) and Zn (Zn²⁺ = full d) are excluded.
• The colour SEEN is the light NOT absorbed. Say: "a d electron absorbs a photon of energy ΔE and is promoted from the ground state to an excited state; the transmitted light gives the colour."
• The chelate effect is entropy-driven. "The number of particles increases, so ΔS is positive; ΔH is close to zero; therefore ΔG is negative."
• Acidify a manganate(VII) titration with SULFURIC acid. HCl would be oxidised to Cl₂; HNO₃ is itself an oxidising agent.

Ligand substitution with ammonia comes in two flavours. A small amount of NH₃ acts as a base and deprotonates the aqua ion to give a precipitate. An excess acts as a ligand and substitutes — which is why the Cu²⁺ precipitate redissolves to the deep blue [Cu(NH₃)₄(H₂O)₂]²⁺.

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Your turn

The big ideas to know

That is the whole of AQA 3.2.5. Press Finish to see your score.