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Chemistry 3% exam weight

Transition Metals

Part of the JAMB UTME study roadmap. Chemistry topic chem-18 of Chemistry.

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Transition Metals

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Transition metals are d-block elements whose atoms (or common ions) have a partially filled d subshell. Electronic configuration pattern: (n−1)d¹⁻¹⁰ ns¹⁻², and they form stable ions by losing the outer s-electrons and some d-electrons. Zn, Cd, Hg are excluded under the IUPAC definition because their d subshell is completely filled (d¹⁰) in the ground state and common ions.

Five diagnostic properties: variable oxidation states (Mn: +2 to +7), coloured compounds (from d–d transitions in split d-orbitals), complex ion formation (central Mⁿ⁺ + ligands → coordinate complexes), catalytic activity (Fe–Haber, V₂O₅–Contact, Pt/Rh–Ostwald, Ni–hydrogenation), and paramagnetism from unpaired d electrons.

IonOxidation stateColour of [M(H₂O)₆]ⁿ⁺
Cu²⁺+2Blue
Fe²⁺+2Pale green
Fe³⁺+3Yellow/brown
Cr³⁺+3Green/violet
Mn²⁺+2Very pale pink

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Defining a Transition Metal (IUPAC)

A transition metal has an incomplete d subshell in the ground state or forms one or more ions with an incomplete d subshell. First-row examples (Sc → Cu) build 3d electrons from 1 to 10 as Z increases. Zn ([Ar] 3d¹⁰ 4s²), Cd, and Hg fail this test: both atom and common ion (Zn²⁺) have d¹⁰, so they are not transition metals in the IUPAC sense — a frequent trap in JAMB MCQs.

Variable Oxidation States

Successive ionisation removes the 4s electrons first, then 3d. The closely spaced d-orbital energies make multiple oxidation states accessible without prohibitive energy cost. Mn exhibits +2, +3, +4, +6, +7; Fe shows +2 and +3; Cu shows +1 and +2; Cr shows +2, +3, +6 (CrO₄²⁻/Cr₂O₇²⁻). Stability of a given state depends on ionisation energy vs lattice/solvation energy; Mn²⁺ (d⁵) and Fe³⁺ (d⁵, half-filled) are extra stable.

Colour and d–d Transitions

In an octahedral ligand field, the five d-orbitals split into a lower t₂g set and an upper e_g set (splitting energy Δ_oct). Electrons absorbing visible photons jump from t₂g to e_g; the transmitted/reflected light carries the complementary colour. Charge-transfer transitions (ligand → metal, LMCT) give very intense colours even when d–d is forbidden — that’s why MnO₄⁻ is purple (Mn⁷⁺, d⁰, no d–d possible) and CrO₄²⁻ is yellow.

Complex Ions and Confirmatory Tests

A complex ion has a central metal (Lewis acid) bonded to ligands (Lewis bases) via dative bonds. Geometry is usually octahedral [ML₆] or tetrahedral/square-planar [ML₄]. Common ligand-substitution example: [Cu(H₂O)₄]²⁺ (blue) + 4NH₃ → [Cu(NH₃)₄]²⁺ (deep blue) + 4H₂O.

Test ionReagentProductObservation
Cu²⁺Excess NH₃[Cu(NH₃)₄]²⁺Deep blue solution
Fe³⁺KSCN[Fe(SCN)]²⁺Blood-red complex
Fe²⁺K₃[Fe(CN)₆]Fe₄[Fe(CN)₆]₃Prussian/Turnbull’s blue
Ni²⁺Dimethylglyoxime[Ni(DMG)₂]Red precipitate

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Crystal Field Theory and Magnetic Moment

In an octahedral field, weak-field ligands (H₂O, Cl⁻, F⁻) give small Δ_oct → high-spin ions (max unpaired electrons); strong-field ligands (CN⁻, NH₃, en) give large Δ_oct → low-spin ions (paired electrons where Δ exceeds pairing energy). The spin-only magnetic moment: μ = √(n(n+2)) BM, where n = number of unpaired electrons.

n (unpaired)μ (BM)
11.73
22.83
33.87
44.90
55.92

Paramagnetism (attracted to a magnet) requires unpaired electrons — diamagnetic Zn²⁺ (d¹⁰) is a useful contrast.

Catalysis — Why d-Block Metals?

Transition metals offer vacant d-orbitals for temporary substrate attachment and variable oxidation states so the metal can be regenerated after each cycle.

ProcessReactionCatalyst
HaberN₂ + 3H₂ ⇌ 2NH₃Fe (Mo/Al₂O₃/K₂O promoters)
Contact2SO₂ + O₂ ⇌ 2SO₃V₂O₅
Ostwald4NH₃ + 5O₂ → 4NO + 6H₂OPt/Rh gauze
HydrogenationC=C + H₂ → C–CFinely divided Ni

Complexometric Titration with EDTA

EDTA⁴⁻ is a hexadentate ligand that wraps any Mⁿ⁺ to give a stable 1:1 octahedral chelate, regardless of metal charge. Therefore moles of EDTA used ≡ moles of metal ion titrated — a key stoichiometric shortcut for Ca²⁺, Mg²⁺, Cu²⁺ and Fe³⁺ hardness/water analyses.

Practice Prompts

  1. Explain, with electronic configurations, why Zn²⁺ is not considered a transition metal ion, even though zinc is a d-block element.
  2. [Cu(H₂O)₄]²⁺ is blue; [Cu(NH₃)₄]²⁺ is deeper blue; [CuCl₄]²⁻ is yellow. Account for the colour changes using ligand field theory.

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