Transition Metals

Which of the following best defines a transition metal in terms of its electron configuration?

• AA d-block element that forms at least one stable ion with a partially filled d sub-shell✓
• BAny element in the d-block of the periodic table
• CA metal that forms coloured compounds in all of its oxidation states
• DAn element whose atoms have a full d sub-shell in the ground state

Answer: A transition metal is a d-block element that forms at least one stable ion with an incomplete (partially filled) d sub-shell. This excludes Sc (only Sc³⁺, d⁰) and Zn (only Zn²⁺, d¹⁰), which are d-block but not transition metals.

What is the ground-state electron configuration of the Fe³⁺ ion (Z = 26)?

• A[Ar] 3d³ 4s²
• B[Ar] 3d⁵✓
• C[Ar] 3d⁴ 4s¹
• D[Ar] 3d⁶

Answer: Fe is [Ar] 3d⁶ 4s². When forming ions the 4s electrons are lost first, then 3d: Fe³⁺ loses both 4s electrons and one 3d electron, giving [Ar] 3d⁵ (a stable half-filled d sub-shell).

Scandium is a d-block element but is NOT classified as a transition metal. Why?

• AIts atoms have no d electrons
• BIt is too reactive to form stable compounds
• CIts only common ion, Sc³⁺, has an empty d sub-shell ([Ar] 3d⁰)✓
• DIt forms only colourless ions, unlike transition metals

Answer: Scandium's only stable ion is Sc³⁺, which has the configuration [Ar] 3d⁰ — an empty d sub-shell. A transition metal must form at least one ion with a partially filled d sub-shell, so Sc is excluded.

What is the shape and bond angle of the complex ion [CuCl₄]²⁻?

• AOctahedral, 90°
• BSquare planar, 90°
• CTrigonal planar, 120°
• DTetrahedral, 109.5°✓

Answer: Chloride ligands are relatively large, so only four fit around the Cu²⁺ centre, giving a coordination number of 4 and a tetrahedral shape with bond angles of 109.5°. (Smaller ligands such as H₂O give octahedral 6-coordinate complexes.)

Ethanedioate (oxalate, C₂O₄²⁻) and 1,2-diaminoethane (en) are both classified as bidentate ligands. What does 'bidentate' mean?

• AEach ligand forms two coordinate bonds to the central metal ion using two lone pairs✓
• BThe ligand carries a 2− charge
• CTwo identical ligand molecules bond to one metal ion
• DThe ligand can be substituted by exactly two water molecules

Answer: A bidentate ligand donates two lone pairs from two different atoms within the same molecule/ion, forming two coordinate bonds to the metal. In en the two N atoms each donate a lone pair; in ethanedioate two O atoms (on different carboxylate groups) each donate a lone pair.

EDTA⁴⁻ forms a 1:1 complex with most metal(II) ions. How many coordinate bonds does one EDTA⁴⁻ ion form, and what is the coordination number of the metal in [M(EDTA)]²⁻?

• A2 coordinate bonds; coordination number 2
• B6 coordinate bonds; coordination number 6✓
• C4 coordinate bonds; coordination number 4
• D8 coordinate bonds; coordination number 8

Answer: EDTA⁴⁻ is hexadentate: it donates six lone pairs (from two N atoms and four O atoms of the carboxylate groups), forming six coordinate bonds. The metal therefore has a coordination number of 6 in [M(EDTA)]²⁻.

In haemoglobin, the Fe²⁺ ion at the centre of the haem group binds oxygen for transport. Carbon monoxide is toxic because it:

• Aoxidises Fe²⁺ to Fe³⁺, which cannot bind oxygen
• Bremoves the haem group from the protein entirely
• Cforms a stronger coordinate bond to Fe²⁺ than oxygen does, so the substitution is effectively irreversible✓
• Dreduces the iron to Fe⁰ metal

Answer: CO acts as a ligand and binds to the Fe²⁺ of haem more strongly than O₂, forming carboxyhaemoglobin. Because this coordinate bond is much stronger, the binding is effectively irreversible, preventing the haemoglobin from carrying oxygen.

Transition metal complexes are often coloured. Which statement correctly explains the origin of the colour?

• AThe d orbitals emit white light when the complex is dissolved
• BElectrons are completely removed from the metal ion by visible light
• CAll wavelengths of visible light are absorbed equally, giving a coloured solution
• DElectrons are promoted between the split d orbitals by absorbing certain frequencies of visible light; the colour seen is the complementary (transmitted) light✓

Answer: Ligands split the 3d orbitals into two energy levels (ΔE). When light is absorbed, a d electron is promoted from the lower to the higher set (a d–d transition). The frequencies absorbed correspond to ΔE; the wavelengths NOT absorbed are transmitted, and we see this complementary colour.

A solution of [Cu(H₂O)₆]²⁺ is pale blue. When concentrated ammonia is added in excess, the colour changes to deep blue as [Cu(NH₃)₄(H₂O)₂]²⁺ forms. Which factor that affects the size of ΔE (and hence the colour) has changed?

• AThe identity of the ligands✓
• BThe oxidation state of the copper
• CThe nuclear charge of the copper
• DThe number of electrons in the 4s orbital

Answer: The oxidation state of Cu remains +2 throughout, so the change must be due to the ligands. Replacing water with the stronger-field ammonia ligand alters ΔE, shifting the wavelengths of light absorbed and changing the colour from pale to deep blue.

Iron is used as a catalyst in the Haber process (N₂ + 3H₂ ⇌ 2NH₃). This is an example of:

• Ahomogeneous catalysis, because the iron dissolves in the gas mixture
• Bheterogeneous catalysis, because the solid iron catalyst is in a different phase from the gaseous reactants✓
• Cautocatalysis, because ammonia catalyses its own formation
• Denzyme catalysis

Answer: The catalyst (solid Fe) is in a different physical state (phase) from the gaseous reactants, so this is heterogeneous catalysis. Reactant molecules adsorb onto active sites on the iron surface, react, and the products desorb.

The reaction between aqueous ethanedioate (C₂O₄²⁻) and manganate(VII) (MnO₄⁻) ions is slow at first but speeds up markedly once some Mn²⁺ has formed. What term describes this behaviour, and which species is responsible?

• AHeterogeneous catalysis; MnO₄⁻ is the catalyst
• BInhibition; C₂O₄²⁻ slows the reaction
• CAutocatalysis; the product Mn²⁺ catalyses the reaction✓
• DHomogeneous catalysis; H⁺ ions are the catalyst

Answer: This is autocatalysis: a product of the reaction (Mn²⁺) acts as a catalyst for that same reaction. As Mn²⁺ builds up, the rate increases, then falls again as the reactants are used up — giving the characteristic S-shaped concentration–time curve.

Why does the substitution of two monodentate water ligands by one bidentate ethanedioate ligand tend to be thermodynamically favourable (the 'chelate effect')?

• AThe enthalpy change is very large and negative
• BThe bidentate ligand forms much weaker bonds, lowering the energy
• CThe reaction is favourable because it decreases the total number of particles
• DThere is a large increase in entropy because more molecules/ions are released into solution than are consumed✓

Answer: Replacing several monodentate ligands with fewer multidentate (chelating) ligands increases the number of free particles in solution (e.g. 1 bidentate replaces 2 monodentate, releasing extra molecules). ΔH is close to zero, so the positive ΔS dominates, making ΔG negative and the chelated complex more stable.