Quick answer: Transition metals are d-block elements that form ions with a partially filled d subshell. They show variable oxidation states, form coloured complex ions with ligands, and act as catalysts. Colour arises when ligands split the d orbitals and electrons absorb light of energy ΔE to jump between them.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.2.5
What makes an element a transition metal?
The d-block elements from titanium to copper fill the 3d subshell after 4s. Their atoms and ions have properties that set them apart from group 1 and group 2 metals: several oxidation states, coloured compounds, catalytic activity and complex ion formation.
A transition element is a d-block element that forms at least one stable ion with a partially filled d subshell.
Scandium forms only Sc³⁺ (3d⁰) and zinc only Zn²⁺ (3d¹⁰), so neither is a transition element even though both are in the d-block. Iron is [Ar] 3d⁶ 4s², and when it ionises the 4s electrons leave first: Fe²⁺ is [Ar] 3d⁶ and Fe³⁺ is [Ar] 3d⁵. Chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹) are exceptions that gain stability from a half-full or full d subshell.
Why do transition metals have variable oxidation states?
The 3d and 4s energies are very close, so a similar amount of energy removes different numbers of electrons. Vanadium shows +2, +3, +4 and +5, and each state has its own colour. Manganese ranges from +2 (pale pink Mn²⁺) to +7 (purple MnO₄⁻).
What is a complex ion?
A complex ion is a central metal ion bonded to ligands by dative covalent bonds. Each ligand donates a lone pair. The number of dative bonds is the coordination number.
| Ligand | Type | Bonds per ligand | Example |
|---|---|---|---|
| H₂O, NH₃, Cl⁻, CN⁻ | monodentate | 1 | [Cu(NH₃)₄(H₂O)₂]²⁺ |
| NH₂CH₂CH₂NH₂ (en), C₂O₄²⁻ | bidentate | 2 | [Ni(en)₃]²⁺ |
| EDTA⁴⁻ | hexadentate | 6 | [Fe(EDTA)]⁻ |
Six ligands usually give an octahedral shape, four give tetrahedral ([CuCl₄]²⁻) or square planar (cisplatin, Pt(NH₃)₂Cl₂), and two give linear ([Ag(NH₃)₂]⁺). The charge on the complex is the sum of the metal ion and ligand charges: Cr³⁺ with four NH₃ and two Cl⁻ gives [Cr(NH₃)₄Cl₂]⁺.
Why are transition metal complexes coloured?
Ligands split the five 3d orbitals into two sets separated by an energy gap ΔE. An electron absorbs a photon of exactly this energy and jumps to the higher set. The remaining wavelengths are transmitted, so the solution shows the complementary colour.
ΔE = hc ÷ λ, with h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹ and λ in metres. For one mole multiply by the Avogadro constant, 6.02 × 10²³ mol⁻¹.
A complex absorbs light of wavelength 650 nm. Calculate ΔE in J per ion and in kJ mol⁻¹.
ΔE = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) ÷ (650 × 10⁻⁹) = 3.06 × 10⁻¹⁹ J. Per mole: 3.06 × 10⁻¹⁹ × 6.02 × 10²³ = 1.84 × 10⁵ J mol⁻¹ = 184 kJ mol⁻¹. Red-orange is absorbed, so the solution looks blue-green.
| Ion | Colour in solution | Ion | Colour in solution |
|---|---|---|---|
| [Cu(H₂O)₆]²⁺ | pale blue | [Cu(NH₃)₄(H₂O)₂]²⁺ | deep blue |
| [CuCl₄]²⁻ | yellow-green | [Fe(H₂O)₆]²⁺ | pale green |
| [Fe(H₂O)₆]³⁺ | yellow-brown | [Co(H₂O)₆]²⁺ | pink |
Changing the ligand, the oxidation state or the coordination number changes ΔE, which changes the colour. This is called ligand substitution: [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O.
How do transition metals act as catalysts?
Heterogeneous catalysts such as iron in the Haber process and vanadium(V) oxide in the contact process provide a surface where reactants adsorb and react; vanadium(V) oxide alternates between +5 and +4. Homogeneous catalysts such as Fe²⁺ or Fe³⁺ in the reaction of iodide with peroxodisulfate switch between oxidation states, avoiding the repulsion between two negative ions: 2Fe²⁺ + S₂O₈²⁻ → 2Fe³⁺ + 2SO₄²⁻, then 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.
Chiamaka titrates 25.0 cm³ of iron(II) solution with 22.40 cm³ of 0.0200 mol dm⁻³ potassium manganate(VII). MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. Calculate the iron(II) concentration.
n(MnO₄⁻) = 0.02240 × 0.0200 = 4.48 × 10⁻⁴ mol; n(Fe²⁺) = 5 × 4.48 × 10⁻⁴ = 2.24 × 10⁻³ mol; c = 2.24 × 10⁻³ ÷ 0.0250 = 0.0896 mol dm⁻³.
Do not call every d-block element a transition element. Zinc and scandium do not form an ion with a partially filled d subshell, so they are not transition metals, and zinc compounds are white.
For ions, remove the 4s electrons first, then 3d. Write the neutral atom, take electrons from 4s, and check that the subshell count equals the oxidation state.
- Transition element: forms an ion with a partially filled d subshell (Sc and Zn are not)
- Fe is [Ar] 3d⁶ 4s²; Cr is [Ar] 3d⁵ 4s¹; Cu is [Ar] 3d¹⁰ 4s¹
- Complex ion = metal ion + ligands joined by dative bonds; coordination number = number of bonds
- Colour: ΔE = hc ÷ λ for the light absorbed
- MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺
Colour, complexes and catalysis in the d-block: frequently asked questions
Why is zinc not a transition metal?
Zinc forms only Zn²⁺, which has a full 3d¹⁰ subshell. A transition element must have an ion with a partially filled d subshell, so zinc compounds are also colourless.
What is a ligand?
A ligand is a molecule or ion with a lone pair that forms a dative covalent bond to a central metal ion. Water, ammonia and chloride ions are common ligands.
Why do complexes have different colours?
The ligands split the d orbitals by an energy gap ΔE. The complex absorbs light of energy ΔE and transmits the other wavelengths, which we see as the complementary colour.
How do transition metals catalyse reactions?
They can change oxidation state, so they form intermediates and provide a lower-energy pathway. Solid catalysts also hold reactants on their surface.
Why does copper(II) solution change colour with ammonia?
Ammonia replaces water ligands to form [Cu(NH₃)₄(H₂O)₂]²⁺. This changes the energy gap, so the colour changes from pale blue to deep blue.
