Last Updated: 11 May 2026
Coordination Compounds is among the highest-yield NEET 2027 chapters in Inorganic Chemistry — 3-4 questions worth 12-16 marks. NCERT Class 12 Ch. 9 has driven 18 NEET questions in the last 5 years, including 4 in NEET 2026.
Werner’s Coordination Theory (1893)
- A metal exhibits two types of valencies — primary (ionisable) and secondary (non-ionisable).
- Primary valencies are satisfied by negative ions; secondary valencies by ligands.
- Every metal has a fixed coordination number (secondary valency).
- Secondary valencies are directional → gives specific geometry.
Key Terminology
- Coordination Sphere: [M(L)n]; written inside square brackets.
- Ligand: Lewis base donating electron pair(s).
- Denticity: number of donor atoms — Monodentate (Cl-, NH3), Bidentate (en, ox), Polydentate (EDTA — hexadentate).
- Chelate: ring-forming polydentate ligand.
- Ambidentate: can bind through different atoms — SCN- (S or N), NO2- (N or O).
IUPAC Nomenclature Rules
- Cation named first.
- Within complex: ligands alphabetically (ignore Greek prefix).
- Prefixes: di-, tri-, tetra- (mono/poly for complex names use bis-, tris-, tetrakis-).
- Metal oxidation state in Roman numerals in parentheses.
- For anionic complex, metal name ends in ‘-ate’ (e.g., ferrate, cuprate).
Isomerism in Coordination Compounds
| Type | Example |
|---|---|
| Ionisation | [Co(NH3)5Br]SO4 vs [Co(NH3)5SO4]Br |
| Hydrate | [Cr(H2O)6]Cl3 vs [Cr(H2O)5Cl]Cl2·H2O |
| Linkage | [Co(NH3)5(NO2)]2+ vs [Co(NH3)5(ONO)]2+ |
| Coordination | [Co(NH3)6][Cr(CN)6] vs [Cr(NH3)6][Co(CN)6] |
| Geometrical (cis-trans) | [Pt(NH3)2Cl2] |
| Optical | [Co(en)3]3+ |
Valence Bond Theory (VBT)
- Inner orbital complex: (n-1)d2sp3 — diamagnetic, low spin (strong field ligands).
- Outer orbital complex: sp3d2 — paramagnetic, high spin (weak field ligands).
- Ligand strength order (Spectrochemical series): I- < Br- < Cl- < F- < OH- < H2O < NH3 < en < CN- < CO.
Crystal Field Theory (CFT) Essentials
- Octahedral splitting: t2g (lower, 3 orbitals) and eg (higher, 2 orbitals). Δ_o = crystal field splitting energy.
- Tetrahedral splitting: e (lower, 2) and t2 (higher, 3); Δ_t ≈ 4/9 × Δ_o.
- CFSE (Crystal Field Stabilisation Energy): = (-0.4 × t2g electrons + 0.6 × eg electrons) × Δ_o.
- Colour: arises from d-d transitions; observed colour is complementary to absorbed.
Important Complexes (NCERT Examples)
- [Fe(CN)6]4-: d2sp3, diamagnetic, hexacyanoferrate(II), used in K4[Fe(CN)6].
- [Ni(CO)4]: sp3, tetrahedral, diamagnetic, Mond’s process.
- [Cu(NH3)4]2+: dsp2, square planar, paramagnetic.
- cis-[PtCl2(NH3)2]: anti-cancer drug (Cisplatin).
- EDTA-Ca2+: water hardness estimation.
Internal Links
FAQ
Average questions from this chapter in NEET?
3-4 questions worth 12-16 marks.
Most-asked subtopic?
Isomerism (linkage + geometrical) — at least 1 question every year.
VBT or CFT — which is more tested?
Both. VBT (hybridisation + magnetic moment) and CFT (splitting + colour) are equally probable.
Practice Quiz
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