Coordination Compounds is Chapter 9 of NCERT Class 12 Chemistry — one of the highest-scoring chapters in NEET Chemistry. In recent NEET papers, 2–3 questions appear consistently from this chapter. Mastering Werner’s theory, IUPAC nomenclature, and types of isomerism guarantees easy marks in NEET 2027.
Werner’s Theory of Coordination Compounds
Alfred Werner (1893) proposed that metals exhibit two types of valency:
| Valency Type | Nature | Feature |
|---|---|---|
| Primary Valency | Ionisable (ionic) | Satisfied by negative ions; shown outside coordination sphere |
| Secondary Valency | Non-ionisable (coordinate) | Satisfied by ligands; defines coordination number |
Example: In [Co(NH₃)₆]Cl₃ — Primary valency = 3 (Cl⁻ ions), Secondary valency = 6 (NH₃ ligands)
Key Definitions
- Coordination entity: Central metal atom + ligands (the complex ion)
- Central atom/ion: Metal that accepts electron pairs from ligands
- Ligands: Ions or molecules that donate electron pairs to central atom
- Coordination number (CN): Total number of ligand donor atoms bonded to central atom
- Coordination sphere: Central atom + ligands enclosed in square brackets
Types of Ligands
| Type | Donor Atoms | Examples |
|---|---|---|
| Monodentate | 1 donor atom | Cl⁻, NH₃, H₂O, CO, CN⁻ |
| Bidentate | 2 donor atoms | en (ethylenediamine), C₂O₄²⁻ (oxalate) |
| Tridentate | 3 donor atoms | dien (diethylenetriamine) |
| Polydentate | Multiple | EDTA (hexadentate) — forms 5 chelate rings |
| Ambidentate | 2 possible donors | SCN⁻ (via S or N), NO₂⁻ (via N or O) |
IUPAC Nomenclature Rules
- Cation is named before anion
- Within coordination sphere: ligands first (alphabetically), then metal
- Anionic ligands end in -o; neutral ligands use common names (aqua, ammine, carbonyl, nitrosyl)
- Oxidation state of metal in Roman numerals in parentheses
- Complex cation uses metal name; complex anion uses metal name + “-ate”
| Complex | IUPAC Name |
|---|---|
| [Co(NH₃)₆]Cl₃ | Hexaamminecobalt(III) chloride |
| [CoCl₂(NH₃)₄]Cl | Tetraamminedichloridocobalt(III) chloride |
| K₄[Fe(CN)₆] | Potassium hexacyanidoferrate(II) |
| [PtCl₂(NH₃)₂] | Diamminedichloridoplatinum(II) |
| [Ni(CO)₄] | Tetracarbonylnickel(0) |
Types of Isomerism in Coordination Compounds
Structural Isomerism
- Linkage isomerism: Ambidentate ligand bonded via different donor atoms — [Co(NO₂)(NH₃)₅]²⁺ vs [Co(ONO)(NH₃)₅]²⁺
- Coordination isomerism: Ligands exchanged between cationic and anionic complex — [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]
- Ionisation isomerism: Ligand and counter ion exchange — [Co(SO₄)(NH₃)₅]Br vs [CoBr(NH₃)₅]SO₄
Stereoisomerism
- Geometrical (cis-trans): Square planar MA₂B₂ type — [Pt(NH₃)₂Cl₂]; Octahedral MA₄B₂ and MA₃B₃ type
- Optical isomerism: Non-superimposable mirror images — [Co(en)₃]³⁺ shows d and l forms
Crystal Field Theory (CFT) — Key Points
CFT explains colour and magnetic properties of coordination compounds:
- Octahedral field: d-orbitals split into t₂g (lower energy, 3 orbitals) and eg (higher energy, 2 orbitals)
- Δo = Crystal field splitting energy (octahedral)
- Strong field ligands (high Δo): CO, CN⁻, NO₂⁻ — low spin, fewer unpaired electrons
- Weak field ligands (low Δo): F⁻, Cl⁻, Br⁻, I⁻, H₂O — high spin, more unpaired electrons
- Spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO
Colour in Coordination Compounds
Colour arises due to d-d transitions. When visible light is absorbed, an electron transitions from t₂g to eg. The colour observed is the complementary colour of absorbed light. Complexes with no d-d transitions (d⁰ or d¹⁰) are colourless — e.g., [Ti⁴⁺] complexes.
Practice MCQs — Coordination Compounds (NEET Level)
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Last updated: April 2026 | NEET Gurukul — Complete NCERT-based preparation for NEET 2027