Coordination Compounds Class 12 Notes: Nomenclature, Isomerism and Crystal Field Theory

Coordination Compounds is worth 4–5 marks in boards and a sure question in JEE/NEET. Naming and CFT are the two areas asked most.

Werner and basic terms

• Central metal + ligands = coordination entity; coordination number = donor atoms attached.
• Ligands: monodentate (NH₃, Cl⁻), bidentate (en, ox²⁻), polydentate (EDTA⁴⁻, hexadentate); ambidentate (NO₂⁻/ONO⁻, SCN⁻/NCS⁻).

IUPAC naming rules

1. Cation first, then anion.
2. Ligands in alphabetical order (ignore di-, tri-); anionic ligands end in -o (chlorido, cyanido).
3. Metal last, with oxidation state in Roman numerals; anionic complex → metal name ends in -ate (ferrate, cuprate).

Examples

• [Co(NH₃)₆]Cl₃ — hexaamminecobalt(III) chloride
• K₄[Fe(CN)₆] — potassium hexacyanidoferrate(II)
• [Pt(NH₃)₂Cl₂] — diamminedichloridoplatinum(II)

Isomerism

• Structural: ionisation, hydrate, linkage, coordination
• Stereo: geometrical (cis/trans in square planar MA₂B₂ and octahedral MA₄B₂, fac/mer in MA₃B₃), optical (cis-[Co(en)₂Cl₂]⁺)

Crystal field theory

• Octahedral: d-orbitals split into t₂g (lower) and e_g (higher), gap Δ₀
• Strong field (CN⁻, CO, en) → pairing, low spin; weak field (I⁻, Br⁻, Cl⁻, F⁻, H₂O) → high spin
• Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO
• Tetrahedral: Δ_t = (4/9) Δ₀; e below t₂
• Colour from d–d transition; magnetic moment μ = √(n(n + 2)) BM
• [Fe(CN)₆]⁴⁻ (d⁶, low spin) diamagnetic; [FeF₆]³⁻ (d⁵ high spin) 5 unpaired

Common mistakes

• Alphabetical order by ligand name, not by number prefix
• Forgetting -ate for anionic complexes
• Using strong-field pairing for H₂O complexes

Related: Electrochemistry notes.

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Frequently asked questions

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They cover every formula, definition and question type that appears in school and board papers for this chapter. Use them with the NCERT examples, exercise questions and the last five years' papers to be fully prepared.

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