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CHEM 08Chemistry3–4 Q/year★★★★ High Priority
d-Block & Coordination Compounds
NCERT Class 12 Ch. 8–9. IUPAC naming, CFT, isomerism, magnetic properties — direct recall questions.
3–4
Questions/Year
12–16
Marks at Stake
Coordination Compounds
Learn IUPAC naming rules methodically. Crystal Field Theory is tested every year — know the splitting patterns.
IUPAC Nomenclature

Order in name: Ligands (alphabetical) before central metal. Cation named before anion.

Ligand names: Anionic ligands get "-o" suffix: F⁻ = fluoro, Cl⁻ = chloro, CN⁻ = cyano, NO₂⁻ = nitro (N-bound), ONO⁻ = nitrito (O-bound), OH⁻ = hydroxo, O²⁻ = oxo, S²⁻ = thio. Neutral: H₂O = aqua, NH₃ = ammine, CO = carbonyl, NO = nitrosyl.

Metal oxidation state: Given in Roman numerals in brackets. Metal names get "-ate" suffix if in anionic complex: Fe = ferrate, Cu = cuprate, Au = aurate, Ag = argentate, Pt = platinate.

Example: [Co(NH₃)₄Cl₂]⁺ = tetraamminedichloridocobalt(III) ion. [Fe(CN)₆]⁴⁻ = hexacyanidoferrate(II) ion.

Crystal Field Theory (CFT)

Octahedral splitting: d-orbitals split into t₂g (lower — dxy, dxz, dyz) and eg (higher — dx²-y², dz²). Δo = crystal field splitting energy. Strong field ligands → large Δo (low spin). Weak field ligands → small Δo (high spin).

Spectrochemical series (weak → strong): I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA < NH₃ < en < CN⁻ < CO.

Crystal field stabilisation energy (CFSE): Energy gained by d electrons going into lower t₂g vs eg. Determines stability. High CFSE → kinetically inert complexes.

Colour: Colour due to d-d transition absorbing specific wavelengths. CuSO₄ is blue → absorbs red. [Ti(H₂O)₆]³⁺ (d¹) is violet → absorbs yellow-green.

Types of Isomerism

Structural isomers:

• Ionisation isomers: [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br (different ions in solution)

• Linkage isomers: [Co(NH₃)₅NO₂]²⁺ (N-linked nitro) vs [Co(NH₃)₅ONO]²⁺ (O-linked nitrito)

• Coordination isomers: [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]

Stereoisomers:

• Geometrical (cis-trans): [Pt(NH₃)₂Cl₂] — cis (Cisplatin, anticancer) vs trans (transplatin)

• Optical isomers: Non-superimposable mirror images. [Co(en)₃]³⁺ shows optical isomerism.

Magnetic Properties

Formula: μ = √(n(n+2)) BM, where n = number of unpaired electrons. BM = Bohr Magneton.

Paramagnetic: Unpaired electrons present → attracted by magnetic field. Most transition metal compounds.

Diamagnetic: All electrons paired → repelled by magnetic field. [Ni(CN)₄]²⁻ (square planar, d⁸, strong field → all paired).

Key examples: [Fe(CN)₆]⁴⁻ (Fe²⁺, d⁶, strong CN⁻ → low spin, 0 unpaired, diamagnetic). [Fe(H₂O)₆]³⁺ (Fe³⁺, d⁵, weak H₂O → high spin, 5 unpaired, μ = √35 ≈ 5.92 BM).

Coordination Chemistry Facts
Spectrochemical Series (order)
Weak field: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO :Strong field
Strong field = large Δo = low spin = fewer unpaired electrons. CO and CN⁻ are always strong field.
Magnetic Moment Values
0 unpaired: 0 BM (diamagnetic)
1 unpaired: 1.73 BM
2 unpaired: 2.83 BM
3 unpaired: 3.87 BM
4 unpaired: 4.90 BM
5 unpaired: 5.92 BM
Formula: μ = √(n(n+2)) BM. Memorise common values for NEET.
Important Complexes
Cisplatin: [Pt(NH₃)₂Cl₂] cis-form, anticancer
Wilkinson's catalyst: [RhCl(PPh₃)₃]
Prussian blue: Fe₄[Fe(CN)₆]₃
Ferrocene: [Fe(η⁵-C₅H₅)₂]
EDTA: hexadentate (6 donors)
Cisplatin is one of the most successful anticancer drugs — its discovery earned Nobel Prize 1979 (but not to Rosenberg — he wasn't awarded)
Ligand Denticity
Monodentate: NH₃, Cl⁻, H₂O, CN⁻
Bidentate: en (ethylenediamine), C₂O₄²⁻
Tridentate: diethylenetriamine (dien)
Hexadentate: EDTA
Chelate: ring-forming multidentate
Chelate complexes are more stable than non-chelate (chelate effect = entropy driven)
Worked Examples
EasyWhat is the IUPAC name of [Co(NH₃)₆]Cl₃?▾
Inner sphere = Co with 6 NH₃ ligands. NH₃ = ammine. Co³⁺ (since 3 Cl⁻ outside). Outer sphere = 3 Cl⁻.
Answer: Hexaamminecobalt(III) chloride
MediumCalculate the magnetic moment of [Fe(H₂O)₆]³⁺ complex.▾
Fe³⁺ has d⁵ configuration. H₂O is a weak field ligand → high spin → all 5 d-orbitals singly occupied → 5 unpaired electrons. μ = √(5(5+2)) = √35 ≈ 5.92 BM.
Answer: μ ≈ 5.92 BM (5 unpaired electrons, paramagnetic)
Mistake DNA
❌ Forgetting "-ate" suffix for anionic complexes
If the OVERALL complex is an ANION, the metal gets an "-ate" suffix. Fe → ferrate, Cu → cuprate, Al → aluminate. If the complex is cationic or neutral, the metal name is unchanged.
Fix: Anion complex → metal name + "-ate". Fe in anion = ferrate. Fe in cation = iron.
❌ Confusing NO₂⁻ linkage isomers
NO₂⁻ is an ambidentate ligand — can bind through N (nitro, -NO₂) or through O (nitrito, -ONO). These form linkage isomers. NEET tests the naming of both.
Fix: Nitro = N-bound (−NO₂). Nitrito = O-bound (−ONO). The binding atom changes the name prefix.
Chapter Intelligence
PYQ Frequency
IUPAC naming: 1–2 Q/year
CFT/spectrochemical series: 1 Q/year
Magnetic moment: 1 Q/year
2026 Prediction
High: Magnetic moment calculation
Expected: Isomerism type identification
Watch: Cisplatin geometry and use
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