d-Block

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

d-Block — Quick Facts

The d-block (transition metals) consists of elements where the (n−1)d orbital is being filled. There are three series:

• 3d series (Sc to Zn): 3d¹⁰4s⁰–2
• 4d series (Y to Cd): 4d¹⁰5s⁰–2
• 5d series (La to Hg, skipping La because 5d starts at Hf): 5d¹⁰6s⁰–2

General Properties of Transition Metals:

Property	Trend/Value
Atomic radii	Decreases across a series (more protons pulling electrons)
Ionisation enthalpy	Increases across series (d-electrons don’t fully shield)
Oxidation states	Multiple (e.g., Mn shows +2 to +7)
Melting point	High (strong metallic bonding due to unpaired d-electrons)
Conductivity	High (delocalised d-electrons)
Density	High (small size + heavy mass)

Most Common Oxidation States:

• Sc: +3 only (no d-electrons)
• Ti: +3, +4 (TiCl₄ is volatile liquid)
• V: +2, +3, +4, +5 (V₂O₅ in contact process)
• Cr: +3, +6 (CrO₃ in chrome plating; Cr₂O₇²⁻ orange, CrO₄²⁻ yellow — pH-dependent interconversion)
• Mn: +2, +4, +7 (KMnO₄ — strong oxidising agent, deep purple)
• Fe: +2, +3 (Fe²⁺ green, Fe³⁺ yellow-brown; Haber-Bosch catalyst)
• Co: +2, +3
• Ni: +2
• Cu: +1, +2 (Cu²⁺ blue in water; Cu⁺ unstable in aqueous solution — disproportionates to Cu + Cu²⁺)
• Zn: +2 only (d¹⁰ — no coloured compounds)

⚡ Exam tip: Zn, Cd, Hg have d¹⁰ configuration throughout — they show only +2 oxidation state (or +1 for Hg in Hg₂²⁺). They are NOT true transition metals by some definitions because they don’t have partially filled d-subshells in their compounds.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

d-Block — NEET/JEE Study Guide

Colour and Electronic Transitions:

Transition metal complexes are coloured because d-d orbital splitting (Δ₀ for octahedral) causes electronic transitions when visible light is absorbed. The colour observed is complementary to the light absorbed.

Ion	Colour of Aqueous Solution	d-electrons
[Ti(H₂O)₆]³⁺	Violet	d¹
[V(H₂O)₆]²⁺	Violet	d³
[Cr(H₂O)₆]³⁺	Blue-green	d³
[Mn(H₂O)₆]²⁺	Pale pink	d⁵
[Fe(H₂O)₆]²⁺	Green	d⁶
[Fe(H₂O)₆]³⁺	Yellow-brown	d⁵
[Co(H₂O)₆]²⁺	Pink	d⁷
[Ni(H₂O)₆]²⁺	Green	d⁸
[Cu(H₂O)₆]²⁺	Blue	d⁹

Δ₀ increases with oxidation state: Fe³⁺ (smaller ion, stronger field) has larger Δ₀ than Fe²⁺.

Magnetic Properties: Number of unpaired electrons determines magnetic moment: μ = √[n(n+2)] BM (Bohr Magnetons)

• d⁵ high spin (e.g., [Mn(H₂O)₆]²⁺): 5 unpaired electrons, μ = 5.92 BM
• d⁶ low spin (e.g., [Co(NH₃)₆]³⁺): 0 unpaired, μ = 0 BM (diamagnetic)

K₃[Cr(C₂O₄)₃]: Complex anion with 3 oxalate ligands (bidentate); all Cr(III) complexes are slow to substitution (kinetically inert) because Cr(III) has d³ configuration.

Lanthanoids (4f series, Z=58–71):

• Lanthanoid contraction: Gradual decrease in ionic radii across the series due to poor shielding by 4f electrons
• Consequence: Zr (4d¹) and Hf (5d¹) have almost identical radii; Nb (4d⁴) and Ta (5d³) are similar
• Cerium: Shows +3 and +4 (CeO₂ = oxidising agent)
• Promethium: Only radioactive lanthanoid
• Lathanoids used in: Camera lenses (La₂O₃ + Nb₂O₅), lighter flints (mischmetal = lanthanoid alloy), nuclear reactors

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

d-Block — Comprehensive Notes

Coordination Compounds — Werner Theory:

Alfred Werner’s coordination theory (Nobel Prize 1913) established that metal ions have two types of valencies:

• Primary valency: Oxidation state (satisfied by ionic bonds)
• Secondary valency: Coordination number (satisfied by coordinate bonds — ligands)

Ligands (Lewis bases donating electron pairs):

• Monodentate: One donor atom (NH₃, H₂O, Cl⁻, CN⁻)
• Bidentate: Two donor atoms (en = ethylenediamine H₂N-CH₂-CH₂-NH₂; oxalate C₂O₄²⁻)
• Polydentate: EDTA⁴⁻ (hexadentate, 4− charge)
• Ambidentate: Can bind through two different atoms (SCN⁻: S or N; NO₂⁻: O or N)

Isomerism in Coordination Compounds:

1. Geometric (cis/trans): [Pt(NH₃)₂Cl₂] — square planar. Cis (same sides, chemotherapy drug cisplatin) vs trans (opposite sides, not effective). Trans isomer cannot have optical isomerism.

2. Optical isomerism: Non-superimposable mirror images. Requires the complex to be chiral (no plane of symmetry). Example: [Co(en)₃]³⁺ (tris-ethylenediamine cobalt(III)) — two enantiomers (Δ and Λ).

3. Ionisation isomerism: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br — different ions in solution.

4. Linkage isomerism: [Co(NH₃)₅ONO]²⁺ (nitrito — O-bonded) vs [Co(NH₃)₅NO₂]²⁺ (nitro — N-bonded).

Crystal Field Theory (CFT): When ligands approach metal ion, d-orbitals split in energy:

• Octahedral: d_z² and d_x²₋ᵧ² (e_g) point directly at ligands → higher energy; d_xy, d_xz, d_yz (t_2g) point between ligands → lower energy. Δ₀ = separation.
• Strong field ligands (CN⁻, CO, NH₃) → large Δ₀ → low spin (paired in t_2g before occupying e_g).
• Weak field ligands (I⁻, Br⁻, S²⁻, H₂O) → small Δ₀ → high spin (maximally unpaired).

Splitting in Tetrahedral Complexes: d_xy, d_xz, d_yz are closer to ligands → higher energy; d_z², d_x²₋ᵧ² are further → lower energy. Δ_t = (4/9) × Δ₀. All tetrahedral complexes are high spin (Δ_t is always small).

Important Industrial Processes (NEET-Relevant):

1. Haber Process (N₂ + 3H₂ → 2NH₃): Iron catalyst with K₂O and Al₂O₃ as promoters. Operates at 400–500°C and 150–200 atm. The iron provides d-electrons to back-donate to N₂, weakening the N≡N bond.

2. Contact Process (2SO₂ + O₂ → 2SO₃): V₂O₅ catalyst (or Pt). SO₃ is absorbed in 98% H₂SO₄ to form oleum (H₂S₂O₇), which is diluted to H₂SO₄.

3. Chrome Plating: Electrolysis of chromic acid (H₂CrO₄) or chromium(VI) solutions. Chromium coats object with a bright, hard, corrosion-resistant layer.

** actinoids (5f series, Z=90–103):**

• All radioactive except Th, Pa, U
• Uranyl ion (UO₂²⁺): Linear, doubly bonded O atoms
• Transuranic elements: Neptunium (93), Plutonium (94), Americium (95) — synthesised in nuclear reactors
• Similar oxidation states (+3, +4, +5, +6, +7) — more complex than lanthanoids

NEET Pattern Analysis: d-Block contributes 2–3 questions per year. Key areas: variable oxidation states (especially Mn, Cr, Fe), colour of complexes (d-d transitions), magnetic properties, coordination isomerism, and crystal field splitting diagrams. The distinction between high spin and low spin, and which ligands are strong/weak field, are frequently tested.

⚡ NEET 2023 Qn: Why is [Ni(CN)₄]²⁻ diamagnetic while [NiCl₄]²⁻ is paramagnetic with 2 unpaired electrons? Answer: CN⁻ is a strong field ligand → large Δ₀ → causes pairing → square planar (dsp²) → all electrons paired (diamagnetic). Cl⁻ is a weak field ligand → small Δ₀ → no pairing → tetrahedral → 2 unpaired electrons (paramagnetic, μ = 2.83 BM).