Properties of transition metals

The properties of transition metals come from their incomplete d sub-level. At A-level there are four to know: they form complex ions, they form coloured ions, they have variable oxidation states, and they show catalytic activity. Like other metals, they also conduct electricity and are usually hard and dense with high melting points.

What is a transition metal?

AQA's A-level specification states that the transition metal characteristics of the elements titanium to copper arise from an incomplete d sub-level in atoms or ions. Across period 4, electrons fill the 3d sub-level, and it is the partly filled 3d sub-level that makes these elements behave the way they do.

When transition metals form ions, they lose their 4s electrons before their 3d electrons. Zinc and scandium sit in the d block, but their only ions have a full or empty 3d sub-level, so they don't show the typical properties. The electron configuration guide shows how to write these configurations.

The four properties of transition metals

1. Complex formation

Transition metal ions bond to ligands. AQA defines a ligand as a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons, and a complex as a central metal atom or ion surrounded by ligands. The co-ordination number is the number of co-ordinate bonds to the central metal atom or ion.

• Water and ammonia are small ligands, so six fit round the metal: [Cu(H₂O)₆]²⁺ is octahedral, with co-ordination number 6.
• Chloride ions are larger, so only four fit: [CuCl₄]²⁻ is tetrahedral, with co-ordination number 4.
• Some complexes are square planar or linear, such as the anticancer drug cisplatin (square planar) and [Ag(NH₃)₂]⁺ (linear).

2. Coloured ions

Most transition metal compounds and solutions are coloured: hydrated copper(II) ions are blue, for example. In a complex, the d orbitals are split into two energy levels. When white light shines through, some wavelengths are absorbed as d electrons are excited from the lower level to the higher one. The rest of the light passes through or is reflected, and that is the colour you see. AQA gives the energy gap as:

For example, a complex that absorbs most strongly at 600 nm has an energy gap of ΔE = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) ÷ (600 × 10⁻⁹) = 3.32 × 10⁻¹⁹ J. Changing the metal's oxidation state, the ligands or the co-ordination number changes ΔE, and so changes the colour. Ions with a full or empty d sub-level, like Zn²⁺ and Sc³⁺, have no d electrons that can jump, so their compounds are white or colourless.

3. Variable oxidation states

AQA states that transition elements show variable oxidation states. The 4s and 3d electrons have similar energies, so different numbers of them can be lost. Manganese, for example, ranges from +2 in Mn²⁺ to +7 in the manganate(VII) ion, MnO₄⁻.

Vanadium shows it beautifully. In a classic Royal Society of Chemistry practical, zinc reduces yellow vanadium(V) step by step:

4. Catalytic activity

Transition metals and their compounds make good catalysts, because they can change between oxidation states and because their surfaces can adsorb reactant molecules. AQA names two kinds:

• Heterogeneous catalysts are in a different phase from the reactants, such as solid iron in the Haber process and vanadium(V) oxide, V₂O₅, in the Contact process.
• Homogeneous catalysts are in the same phase, such as Fe²⁺ ions catalysing the reaction between iodide ions and peroxodisulfate ions in solution.

In that second reaction, both reactants are negative ions, so they repel each other and react slowly. Fe²⁺ provides a route with two steps, each between oppositely charged ions:

1. S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
2. 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

The Fe²⁺ is made again in the second step, so it is a catalyst. Adding the steps gives the overall reaction, S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂.

Summary of the properties

Common mistakes

• Removing 3d electrons first. Transition metals lose their 4s electrons first: Fe²⁺ is [Ar] 3d⁶, not [Ar] 3d⁴ 4s².
• Saying the colour is the light absorbed. The colour you see is the light that is not absorbed.
• Calling zinc a transition metal. It is in the d block, but Zn²⁺ has a full d sub-level.
• Defining a ligand as an ion only. A ligand can be a molecule (H₂O, NH₃) or an ion (Cl⁻, CN⁻). What matters is the lone pair it donates.
• Mixing up the catalyst types. Heterogeneous means a different phase from the reactants; homogeneous means the same phase.

To practise transition metal questions in a short timed paper, try today's A-level Chemistry: Year 2 paper.

Read next

Chemistry · Atoms and the periodic tabled orbitals: shapes, filling order and splittingWhat d orbitals are, the shapes of all five, how 3d and 4s fill in transition metals, and how d orbital splitting gives complexes their colour.