Properties of transition metals

The essentials

Partly filled d orbitals give transition metals and their compounds a set of characteristic properties: paramagnetism, coloured ions, the ability to form complexes, catalytic activity and the formation of interstitial compounds and alloys.

Key points

• Magnetism: substances with unpaired electrons are paramagnetic. Spin-only magnetic moment μ = √[n(n+2)] BM, where n is the number of unpaired electrons. For n = 1, 2, 3, 4, 5, μ ≈ 1.73, 2.83, 3.87, 4.90, 5.92 BM.
• Colour: when visible light falls on the ion, an electron is excited from a lower to a higher d orbital (d–d transition). The colour seen is complementary to the light absorbed. Ions with d⁰ or d¹⁰ configuration (Sc³⁺, Ti⁴⁺, Zn²⁺, Cu⁺) are usually colourless.
• Complex formation: small, highly charged ions with vacant d orbitals form complexes such as [Fe(CN)₆]³⁻ and [Cu(NH₃)₄]²⁺.
• Catalysis: variable oxidation states and the ability to adsorb reactants make them good catalysts. Examples: V₂O₅ in the contact process, finely divided Fe in the Haber process, Ni in hydrogenation.
• Interstitial compounds: small atoms (H, C, N) trapped in metal lattices, e.g. TiC, Fe₃H. They are hard, have high melting points, keep metallic conductivity and are chemically inert.
• Alloys form readily because the atoms have similar radii; e.g. brass (Cu–Zn), bronze (Cu–Sn).

Exam tips

• Mn²⁺ and Fe³⁺ (both d⁵) have the maximum unpaired electrons (5) among common 3d ions.

Common mistakes

• Counting electrons in the neutral atom instead of the ion when finding n.