Coordination Compounds Revision Notes (JEE Main)

Werner's Theory

• Primary valency = oxidation state, satisfied by anions outside sphere.
• Secondary valency = coordination number, satisfied by ligands (neutral or anionic).

IUPAC Nomenclature

1. Cation first, then anion.
2. Ligands alphabetically before central metal.
3. Prefixes: di, tri (mono ligands) OR bis, tris (for complex names).
4. Oxidation state of metal in Roman numerals.
5. If complex is anion, metal name ends in "-ate" (e.g., ferrate, cuprate, plumbate).

Common Ligands & Denticity

• Monodentate: NH₃, H₂O, CN⁻, Cl⁻, CO.
• Bidentate: en (ethylenediamine), ox (C₂O₄²⁻), gly.
• Hexadentate: EDTA⁴⁻ (used in complexometric titration).

Effective Atomic Number

• EAN = Z − oxidation state + 2×CN.
• Rule: complexes tend towards nearest noble-gas EAN (e.g., Ni(CO)₄ → 36).

Crystal Field Theory (CFT)

• Octahedral splitting: t₂g (lower) below e_g (higher), gap Δ₀.
• Tetrahedral splitting: e (lower) below t₂ (higher), Δ_t = 4Δ₀/9.
• Strong field (CN⁻, CO, NH₃ high) → low spin; weak field (I⁻, Br⁻, F⁻, H₂O) → high spin.
• Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ ≈ CO.
• CFSE (octahedral) = (−0.4 t₂g count + 0.6 e_g count)Δ₀ + P·(extra pairs).

Magnetic Moment

• Spin-only: $\mu = \sqrt{n(n+2)}$ BM, n = unpaired e⁻.
• 1e⁻ → 1.73 BM, 2 → 2.83, 3 → 3.87, 4 → 4.90, 5 → 5.92.

Isomerism

• Structural: ionisation, hydrate/solvate, linkage (NO₂ vs ONO), coordination.
• Stereo: geometrical (cis/trans in MA₂B₂ square planar & octahedral MA₄B₂), optical (chelates like [Co(en)₃]³⁺).

Colour

• d-d transitions absorb visible → complementary colour seen.
• d⁰ (TiO₂, ZnO), d¹⁰ (Zn²⁺, Cu⁺) → colourless.

Applications

• Chelation therapy (EDTA), Hb (Fe-porphyrin), chlorophyll (Mg-porphyrin), cis-platin (anti-tumour).

Practice MCQs

Q1. Oxidation state of Cr in [Cr(H₂O)₆]Cl₃ is A) +2 B) +3 C) +4 D) +6

Q2. Which is a bidentate ligand? A) NH₃ B) CN⁻ C) en D) H₂O

Q3. Magnetic moment of Fe³⁺ (high spin) is closest to A) 1.73 B) 2.83 C) 4.90 D) 5.92

Q4. [Ni(CN)₄]²⁻ is A) Tetrahedral, paramagnetic B) Square planar, diamagnetic C) Octahedral, paramagnetic D) Trigonal bipyramidal

Q5. Colour of an aqueous solution of [Cu(H₂O)₄]²⁺ is A) Colourless B) Blue C) Green D) Yellow

Q6. CFSE of a d⁶ strong-field octahedral complex (in Δ₀) is A) −0.4 B) −1.6 C) −2.4 D) −2.0

Q7. Number of geometrical isomers in [Co(NH₃)₄Cl₂]⁺ is A) 1 B) 2 C) 3 D) 4

Q8. Which is optically active? A) trans-[Co(en)₂Cl₂]⁺ B) cis-[Co(en)₂Cl₂]⁺ C) [Co(NH₃)₆]³⁺ D) [Ni(CO)₄]

Q9. EAN of Fe in Fe(CO)₅ is A) 34 B) 35 C) 36 D) 38

Q10. In spectrochemical series, strongest field ligand is A) I⁻ B) F⁻ C) H₂O D) CN⁻

Answers: 1-B (Three Cl outside → +3.), 2-C (Ethylenediamine has two N-donor sites.), 3-D (5 unpaired e⁻ in 3d⁵.), 4-B (Strong field CN⁻ pairs electrons in dsp² Ni²⁺.), 5-B (d⁹ d-d absorbs red-orange.), 6-C ((6×−0.4)+2P convention; ignoring P: −2.4Δ₀.), 7-B (cis and trans.), 8-B (cis-form lacks plane of symmetry.), 9-C (26 − 0 + 10 = 36 (Kr).), 10-D (CN⁻/CO are the strongest field ligands.)

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