CBSE Class 12 Chemistry: Coordination Compounds — Notes 2026

Basic Terms

• Central metal ion: transition metal; accepts electron pairs from ligands
• Ligand: atom, ion or molecule donating lone pair to metal; monodentate, bidentate, polydentate
• Coordination sphere: central metal + ligands in square brackets; counter ions outside brackets

IUPAC Nomenclature

• Name ligands first (alphabetically, with prefixes di, tri, bis, tris for polydentate), then metal
• Anion names: Cl⁻ = chlorido, Br⁻ = bromido, CN⁻ = cyanido, NH₃ = ammine, H₂O = aqua, NO₂⁻ = nitrito
• Example: [Co(NH₃)₄Cl₂]⁺ = tetraaminedichloridocobalt(III) ion

Isomerism in Coordination Compounds

• Structural isomers: ionisation isomers ([Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅(SO₄)]Br); linkage isomers (NO₂ vs ONO ligand)
• Stereoisomers: geometrical (cis/trans in square planar and octahedral) and optical (non-superimposable mirror images)
• Cis-platin [PtCl₂(NH₃)₂]: anticancer drug; trans-platin: no activity; geometrical isomerism critical to function

Valence Bond Theory (VBT)

• Metal uses empty orbitals (hybridised) to accept electron pairs from ligands
• Octahedral: d²sp³ (inner sphere) or sp³d² (outer sphere); tetrahedral: sp³; square planar: dsp²
• Limitation: doesn't explain colour and magnetic properties quantitatively; assumes pure covalent bond

Crystal Field Theory (CFT)

• Ligands are point charges; electrostatic interaction with metal d-orbitals; no covalent bond assumed
• Octahedral field: d-orbitals split into t₂g (lower energy, dxy, dxz, dyz) and eg (higher, dx²-y², dz²)
• Crystal field splitting energy Δ₀; low spin (strong field ligands, high Δ₀) vs high spin (weak field, low Δ₀)

Colour and Magnetism

• Colour: d-d transition; electron absorbs complementary colour photon to jump from t₂g to eg
• Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO (weak to strong field)
• Magnetic moment: unpaired electrons → paramagnetic; μ = √(n(n+2)) BM where n = unpaired electrons

CBSE Board Focus

• Coordination compounds: 5–7 marks; IUPAC naming, isomerism types, CFT explanation of colour
• Numerical: oxidation state of metal, magnetic moment calculation
• Draw: tetrahedral, square planar, octahedral structures; label coordinate bond