d-Block

d-Block

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your JEE Main shift.

The d-block spans Groups 3–12 with the general configuration (n–1)d¹⁻¹⁰ ns¹⁻². Two electrons break the pattern: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹, because half-filled and fully-filled d-subshells are extra stable.

• Variable oxidation states arise since (n–1)d and ns orbitals sit at nearly the same energy.
• Coloured compounds appear when d-d transitions absorb visible light (Δₒ ≈ 10,000–30,000 cm⁻¹).
• Spin-only magnetic moment: μ_so = √(n(n+2)) BM, where n = unpaired electrons.
• Zn, Cd, Hg are d¹⁰ and show no d-d colour and a fixed +2 state.
• Charge-transfer colours (MnO₄⁻ violet, CrO₄²⁻ yellow) have no unpaired electrons.

High-yield facts for JEE Main: Cr/Cu configurations, the μ_so formula, and identification of colour in KMnO₄ and K₂Cr₂O₇.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students working through the syllabus in a few weeks.

Electronic Configuration and the Two Exceptions

Across a d-series the electron enters (n–1)d, while ns fills first because of its lower energy in neutral atoms. Two ions of merit deviate: Cr = [Ar] 3d⁵ 4s¹ and Cu = [Ar] 3d¹⁰ 4s¹. The driving force is exchange-energy stabilisation of half-filled (d⁵) and completely-filled (d¹⁰) subshells, which trumps the small energy cost of promoting one 4s electron.

Atomic Size, IP and Oxidation States

Across a period, 3d radii shrink only weakly because added d electrons shield the nuclear charge poorly; radii then slightly rise near the end of the series. The 4d and 5d rows have nearly identical radii — the lanthanoid-contraction analogue. Ionisation enthalpies show irregular jumps because each successive electron may come from a different subshell. Oxidation states span from +2 to +7 in 3d (Mn shows +2, +4, +6, +7), and stability peaks at d⁵ (Mn²⁺, Fe³⁺) and d¹⁰ (Cu⁺, Zn²⁺).

Colour, Magnetism and CFSE

Octahedral crystal-field splitting (Δₒ) raises t₂g and lowers eₘ; electrons fill t₂g first. High-spin complexes arise when Δₒ < P (pairing energy); low-spin when Δₒ > P, relevant for d⁴–d⁷. Tetrahedral splitting Δₜ = (4/9)Δₒ is small, so tetrahedral complexes are always high-spin with negligible CFSE.

Property	3d series behaviour	Reason
Atomic radius	Shrinks slowly, rises at end	Poor d-electron shielding
IP trend	Irregular (Mn > Fe, Zn > Cu)	Subshell changeover
Common O.S.	+2, +3 (extend to +7)	(n–1)d, ns energy similarity
Colour	Visible-range d-d absorptions	Δₒ ≈ 10,000–30,000 cm⁻¹

Coordination Compounds and Standard Electrode Potentials

Werner’s theory defines primary (ionisable) and secondary (non-ionisable) valencies. IUPAC names ligands first with -o endings (chlorido, sulphato, oxalato), metal last as -ate for anionic complexes (e.g., hexacyanidoferrate(III) for [Fe(CN)₆]³⁻). Standard potentials M²⁺/M are less negative than predicted by IP alone because of low hydration enthalpy and high sublimation energy; Mn²⁺/Mn and Zn²⁺/Zn are anomalously negative for the same d⁵/d¹⁰ reasons.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for the JEE Main aspirant on a multi-month plan.

Edge Cases That Examiners Test

1. Group 12 status. Zn, Cd, Hg have d¹⁰ in ground state and common oxidation state, so a strict “incompletely filled d” definition excludes them; NCERT still groups them with d-block for teaching.
2. Charge-transfer vs d-d transitions. MnO₄⁻ is violet despite Mn(VII) being d⁰ — colour comes from ligand → metal charge transfer, not d-d. Same logic for yellow CrO₄²⁻ and red Fe(SCN)₆³⁻.
3. Square-planar d⁸. Pd²⁺, Pt²⁺, Au³⁺ ions prefer dsp² square-planar geometry (e.g., [Ni(CN)₄]²⁻ is square planar but [NiCl₄]²⁻ is tetrahedral because Cl⁻ is a weak-field ligand).
4. Ferromagnetism. Fe, Co, Ni show bulk ferromagnetism not predicted by the spin-only formula — it arises from aligned domains in the metal lattice, a non-coordination phenomenon.

Worked CFSE Computation

For [Fe(CN)₆]⁴⁻ (Fe²⁺ = d⁶, CN⁻ is strong field, Δₒ > P → low spin):

Step	Value
Electrons in t₂g	6
CFSE	(6 × –0.4Δₒ) + (2P) = –2.4Δₒ + 2P

For [Fe(H₂O)₆]²⁺ (H₂O weak field, high-spin d⁶): t₂g⁴ eg² → CFSE = (4 × –0.4) + (2 × +0.6) = –0.4Δₒ, with 0 pairing penalty.

Stability, Isomerism and the Irving-Williams Order

For M²⁺ with a given ligand, stability follows Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺ — Cu²⁺ is anomalously high because of strong Jahn-Teller distortion and the d⁹ configuration. Isomerism checklist for any complex-ion question:

• Structural: linkage (NO₂⁻ vs SCN⁻), ionisation, coordination, hydrate.
• Stereoisomerism: cis-trans (MA₄B₂, MA₂B₂C₂), fac-mer (MA₃B₃), optical (cis-MA₂B₂C₂ and Δ/Λ tris-bidentate).

JEE Main Strategy

Aspect	What JEE Main typically asks
Weight	~3% → 1 question/shift
Format	Single MCQ, occasionally 2 from coordination compounds
Time budget	≤ 2 minutes
Common traps	Cr/Cu config, μ_so unit confusion, d⁰ vs charge-transfer colour

Two Practice Prompts

1. A complex has μ_so = 2.83 BM and contains Co in an octahedral field with a strong-field ligand. Identify the oxidation state and electron configuration.
2. Predict whether [CoF₆]³⁻ and [Co(CN)₆]³⁻ are high-spin or low-spin, and state which absorbs at longer wavelength.

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