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 that appears in the d-block of the periodic table, whatever ions it goes on to form
• CA metal that forms coloured compounds in every one of its oxidation states, including the zero state
• DAn element whose atoms have a full d sub-shell (d¹⁰) 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?

• AIt has no d electrons of any sort in its ground state atoms
• BIt is far too reactive to form stable compounds in the laboratory or in nature
• CIts only common ion, Sc³⁺, has an empty d sub-shell ([Ar] 3d⁰)✓
• DIt forms only colourless ions, and colour is what defines a transition metal

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 an overall 2− charge, as ethanedioate does
• CTwo identical ligand molecules bond to the same metal ion at the same time, forming a pair
• DThe ligand can be substituted by exactly two water molecules (an aqua complex)

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 the Fe²⁺ at the centre of the haem group to Fe³⁺, which can then no longer bind an oxygen molecule
• Bremoves the haem group from the protein entirely, so that no oxygen or CO₂ can be carried
• Cforms a stronger coordinate bond to Fe²⁺ than oxygen does, so the substitution is effectively irreversible✓
• Dreduces the iron all the way to Fe⁰ metal, which then precipitates out of the blood as fine (metallic) particles

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?

• AAbsorption of white light emitted internally by the metal's d orbitals, partially filtered by the surrounding ligands, produces the observed colour
• BComplete photo-ionisation removes electrons from the metal ion, and the energy released as those electrons depart produces the observed colour
• CEvery wavelength of visible light is absorbed equally strongly by the complex, right across the visible spectrum
• 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 to give a single phase
• Bheterogeneous catalysis, because the solid iron catalyst is in a different phase from the gaseous reactants✓
• Cautocatalysis, because the ammonia produced catalyses its own further formation
• Denzyme catalysis, because the iron surface behaves in the same way as a protein enzyme

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; the MnO₄⁻ ions adsorb onto the vessel surface
• BInhibition; C₂O₄²⁻ slows the reaction
• CAutocatalysis; the product Mn²⁺ catalyses the reaction✓
• DHomogeneous catalysis; H⁺ ions from the acid act as 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 on chelation is very large and negative
• BThe bidentate ligand forms much weaker dative bonds, which lowers the overall energy
• CThe reaction is favourable because it decreases the total number of particles present in the solution, giving a more ordered system
• 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.

What defines a transition metal?

• AIt forms at least one ion with a completely full d subshell
• BIt has a partially filled d subshell in its atomic state only
• CIt occupies the d block of the periodic table without exception
• DIt forms at least one ion with a partially filled d subshell✓

Answer: The definition is about the ION, which is why scandium and zinc sit in the d block but are not transition metals — scandium's only ion is d⁰ and zinc's is d¹⁰. The partly filled d subshell explains colour, variable oxidation state and catalysis.

Why are transition metal complexes coloured?

• Ad electrons absorb light and leave the complex entirely
• BThe ligands themselves absorb the visible light
• Cd electrons absorb light and move to a higher d level✓
• DThe metal ion emits light of a characteristic frequency

Answer: Ligands split the d orbitals into two energy levels, and light of the matching frequency promotes an electron between them. The colour seen is what remains after that frequency is absorbed.

What is a ligand?

• AA species accepting an electron pair from a metal ion
• BA species that removes electrons from a metal ion
• CA species donating an electron pair to a metal ion✓
• DA species that changes the metal's oxidation state

Answer: Ligands form co-ordinate bonds by donating a lone pair into empty metal orbitals. Monodentate ligands such as water donate one pair; bidentate and multidentate ligands donate several.

Why is the chelate effect entropically favourable?

• ASeveral multidentate ligands replace one monodentate one
• BThe reaction releases a large amount of heat energy
• CThe complex formed contains fewer total particles
• DOne multidentate ligand replaces several monodentate ones✓

Answer: Replacing six water molecules with three bidentate ligands raises the number of free particles, so entropy increases and ΔG becomes more negative. The enthalpy change is usually very small, so entropy decides.

Why do transition metals make good catalysts?

• AFixed oxidation states let them readily accept and donate electrons
• BThey have completely full d subshells in every one of their ions
• CVariable oxidation states let them accept and donate electrons✓
• DThey form only ionic compounds with strong lattices

Answer: Being able to change oxidation state lets a transition metal form intermediates in a heterogeneous or homogeneous route with lower activation energy, then return to its original state. Iron in the Haber process is the standard example.

What shape is a complex with six monodentate ligands?

• ATetrahedral, with bond angles of 109.5 degrees
• BSquare planar, with bond angles of 90 degrees
• CLinear, with bond angles of 180 degrees
• DOctahedral, with bond angles of 90 degrees✓

Answer: Six ligands arrange themselves as far apart as possible, giving an octahedron. Four small ligands usually give a tetrahedron, and two ligands give a linear complex such as Tollens' reagent.

What colour change is seen when copper(II) sulfate solution is treated with excess ammonia?

• ADeep blue to pale blue as ligand substitution occurs
• BPale blue to green as the copper is oxidised
• CPale blue to deep blue as ligand substitution occurs✓
• DGreen to colourless as the complex decomposes

Answer: A little ammonia first gives a pale blue precipitate of copper(II) hydroxide, which redissolves in excess as ammonia replaces four water ligands to give the deep blue tetraamminediaquacopper(II) ion.

Why can cis-platin act as an anti-cancer drug?

• AIt binds to RNA and prevents proteins being translated
• BIt dissolves the cell membrane of the cancerous cells
• CIt binds to DNA and prevents cancer cells replicating✓
• DIt supplies platinum ions which catalyse cell repair

Answer: The two chloride ligands are replaced by nitrogen atoms on guanine bases, forming a cross-link that stops the DNA strands separating. The trans isomer has the wrong geometry and is ineffective.

What is meant by a bidentate ligand?

• AIt donates two lone pairs to the central metal ion✓
• BIt donates one lone pair to two separate metal ions
• CIt carries a charge of exactly two on the ligand
• DIt can be replaced by exactly two water molecules

Answer: A bidentate ligand such as ethanedioate or 1,2-diaminoethane binds through two donor atoms, forming a ring with the metal. Three of them fill an octahedral complex.