Covers NCERT Class 12 Chemistry, Chapter "Coordination Compounds" — Werner's theory, nomenclature, isomerism, VBT and Crystal Field Theory. A consistently very-high-yield chapter for NEET.
IUPAC nomenclature of complexes, calculating EAN and oxidation state of the central metal, CFT splitting (Δ₀ vs Δₜ) and high-spin/low-spin configuration, magnetic behaviour from unpaired electrons, isomerism (geometrical & optical in octahedral/square planar complexes), and the difference between double salts and coordination compounds (ionisation in water).
Werner's theory: metals show two types of valency — primary (ionisable, satisfied by anions) and secondary (non-ionisable, satisfied by ligands, fixed = coordination number, directional in space).
A ligand donates at least one electron pair to the central atom/ion. Denticity = number of donor atoms from one ligand binding the same metal — monodentate (NH₃), bidentate (en, ox²⁻), polydentate (EDTA⁴⁻, hexadentate).
Coordination number = number of ligand donor atoms directly bonded to the central metal — 4 usually gives tetrahedral or square planar, 6 usually gives octahedral geometry.
Nomenclature order inside the coordination sphere: ligands named alphabetically (ignoring multiplying prefixes), anionic ligands end in "-o", then the metal name, then its oxidation state in Roman numerals in brackets. Complex anions end in "-ate".
Two structurally distinct classes of isomerism dominate NEET questions: (a) geometrical (cis/trans, fac/mer) — needs at least two different ligand types on a square planar or octahedral centre; (b) optical — non-superimposable mirror images, common in octahedral complexes with bidentate ligands.
Valence Bond Theory: hybridisation of the metal decides geometry — sp³ (tetrahedral), dsp² (square planar), sp³d² (outer orbital octahedral, high spin), d²sp³ (inner orbital octahedral, low spin).
Crystal Field Theory: in an octahedral field, the d-orbital set splits into a lower t₂g (3 orbitals) and higher eg (2 orbitals) set, separated by Δ₀. Strong-field ligands (CN⁻, CO) give large Δ₀ → pairing favoured → low spin; weak-field ligands (F⁻, H₂O, halides) give small Δ₀ → high spin.
I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ ≈ CO (weak → strong field, increasing Δ₀).
A double salt (e.g. Mohr's salt, potash alum) ionises completely into all its simple ions in water and loses its identity; a coordination compound retains its complex ion largely intact in solution (e.g. [Fe(CN)₆]⁴⁻ does not test positive for free Fe²⁺ or free CN⁻).
| Hybridisation | Geometry | Spin type | Example |
|---|---|---|---|
| sp³ | Tetrahedral | — | [NiCl₄]²⁻ |
| dsp² | Square planar | Low spin | [Ni(CN)₄]²⁻ |
| sp³d² | Octahedral (outer orbital) | High spin | [FeF₆]³⁻ |
| d²sp³ | Octahedral (inner orbital) | Low spin | [Fe(CN)₆]³⁻ |
Original questions modelled on recurring NEET question types — not verbatim reproductions of any official paper.
Scaled from direct NCERT application (Q1–Q6) to mixed NEET-level difficulty (Q7–Q15).
Purely a recall device — not a reinterpretation of the source stories.
Picture the central metal ion as a king holding court, like Indra. Only a fixed number of courtiers are allowed in the inner circle at once — that fixed number is the coordination number (commonly 4 or 6). Each courtier (ligand) offers a pair of electrons as tribute — a coordinate bond, donor to acceptor, one-directional in origin even though the resulting bond is as strong as any covalent bond. A courtier skilled enough to serve the king from two posts at once (two donor atoms, like ethylenediamine) is the bidentate ligand; one who can serve from six posts (EDTA) is prized above all — a hexadentate advisor.
In an octahedral court, six courtiers stand along the x, y and z axes. Two of the king's own attendants (the eg orbitals) happen to be seated exactly along those axes and feel the crowding most; three others (t₂g) sit in between the axes and are comparatively undisturbed. That difference in "how crowded each seat feels" is Δ₀. Persuasive courtiers (strong-field ligands like CN⁻) crowd the king's attendants enough to force them to pair up in the calmer seats rather than spread out — giving a low-spin arrangement.
"I Bring Cats Fully Home, Watching Nightly, Eating Cheese, Constantly" → I⁻, Br⁻, Cl⁻, F⁻, H₂O, NH₃, en, CN⁻/CO — weak field to strong field, left to right.