JEE Chemistry 2027 — Coordination Compounds is one of the most conceptual chapters in JEE Chemistry from NCERT Class 12 Chapter 9. It contributes approximately 3–4 questions every year. The chapter covers Werner’s theory, IUPAC naming, VBT, Crystal Field Theory, and isomerism. This comprehensive guide will help you master all key concepts and score full marks from this chapter.
1. Basic Terminology
Before studying coordination compounds, it is essential to understand the key terminology:
- Complex ion / Coordination entity: A central metal atom or ion surrounded by a fixed number of ligands. Example: [Co(NH₃)₆]³⁺
- Central metal atom/ion: The metal atom/ion at the centre of the complex that accepts electron pairs from ligands. Generally a transition metal.
- Ligands: Ions or molecules that donate electron pairs to the central metal. Lewis bases. Examples: NH₃, Cl⁻, H₂O, CN⁻, en (ethylenediamine).
- Coordination number (CN): The total number of donor atoms (not ligands) directly bonded to the central metal. Example: In [Co(en)₃]³⁺, en is bidentate so CN = 6.
- Coordination sphere: The central metal and its attached ligands, enclosed in square brackets.
- Counter ions: Ions outside the coordination sphere that balance the charge of the complex. Example: In [Co(NH₃)₆]Cl₃, the 3 Cl⁻ are counter ions.
- Oxidation state: The charge on the central metal if all ligands were removed with their electron pairs.
2. Types of Ligands
| Type | Donor Atoms | Examples |
|---|---|---|
| Monodentate (unidentate) | 1 | NH₃, Cl⁻, H₂O, CN⁻, NO₂⁻, CO, F⁻, Br⁻ |
| Bidentate | 2 | en (ethylenediamine), C₂O₄²⁻ (oxalate), 2,2′-bipyridyl |
| Tridentate | 3 | diethylenetriamine (dien) |
| Hexadentate | 6 | EDTA (ethylenediaminetetraacetate) |
| Ambidentate | 1 (can bind via two different atoms) | NO₂⁻ (via N or O), SCN⁻ (via S or N) |
Chelating ligands are poly-dentate ligands that form ring structures (chelate rings) with the central metal. Chelate complexes are more stable than complexes with monodentate ligands (chelate effect).
3. IUPAC Nomenclature of Coordination Compounds
The IUPAC naming rules for coordination compounds are frequently tested in JEE Main. The key rules are:
- Cation before anion: Name the cation first, then the anion.
- Ligands before metal: Within the coordination sphere, name ligands first (in alphabetical order, ignoring prefixes), then the central metal.
- Anionic ligands end in -o: Cl⁻ → chlorido, CN⁻ → cyanido, NO₂⁻ → nitrito
- Neutral ligands use their names: NH₃ → ammine, H₂O → aqua, CO → carbonyl, NO → nitrosyl
- Prefixes for multiple ligands: di, tri, tetra, penta, hexa for simple ligands; bis, tris, tetrakis for complex/polydentate ligands
- Oxidation state of metal: Written in Roman numerals in parentheses after metal name
- Anionic complexes end in -ate: [Fe(CN)₆]⁴⁻ → hexacyanidoferrate(II)
IUPAC Naming Examples
| Formula | IUPAC Name |
|---|---|
| [Co(NH₃)₆]Cl₃ | Hexaamminecobalt(III) chloride |
| [PtCl₂(NH₃)₂] | Dichloridodiammineplatinum(II) |
| [Fe(CN)₆]⁴⁻ | Hexacyanidoferrate(II) ion |
| [Cr(NH₃)₃(NO₂)₃] | Triamminetrinitrito-N-chromium(III) |
| K₄[Fe(CN)₆] | Potassium hexacyanidoferrate(II) |
| [Co(en)₂Cl₂]Cl | Bis(ethylenediamine)dichloridocobalt(III) chloride |
4. Bonding in Coordination Compounds
Werner’s Theory (1893)
Alfred Werner proposed the theory of coordination compounds:
- Metals have two types of valencies: primary valency (ionisable, gives oxidation state) and secondary valency (non-ionisable, gives coordination number)
- Primary valency is satisfied by negative ions; secondary valency is satisfied by both negative ions and neutral molecules
- Secondary valency is always satisfied — it determines the geometry of the complex
Valence Bond Theory (VBT)
VBT explains bonding by considering the hybridisation of metal orbitals and overlap with ligand orbitals:
| Coordination Number | Hybridisation | Geometry |
|---|---|---|
| 2 | sp | Linear |
| 4 | sp³ | Tetrahedral |
| 4 | dsp² | Square planar |
| 6 | sp³d² (outer) | Octahedral |
| 6 | d²sp³ (inner) | Octahedral |
Inner orbital complex: Uses (n-1)d orbitals for hybridisation (d²sp³). Formed with strong field ligands. Diamagnetic (paired electrons). Example: [Fe(CN)₆]⁴⁻
Outer orbital complex: Uses nd orbitals (sp³d²). Formed with weak field ligands. Paramagnetic. Example: [FeF₆]³⁻
5. Crystal Field Theory (CFT)
CFT is an ionic model that considers the effect of electrostatic field of ligands on the d-orbitals of the central metal. In a free metal ion, all five d-orbitals are degenerate (equal energy). In a complex, the ligand field splits them.
Octahedral Field Splitting
In an octahedral complex, the d-orbitals split into two sets:
- eg set (higher energy): dx²-y², dz² — pointing toward ligands
- t₂g set (lower energy): dxy, dxz, dyz — pointing between ligands
The energy difference is Δo (crystal field splitting energy). The t₂g orbitals are stabilised by 0.4Δo; eg orbitals are destabilised by 0.6Δo.
Strong Field vs Weak Field Ligands
Spectrochemical Series (partial, increasing field strength):
I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NCS⁻ < NH₃ < en < NO₂⁻ < CN⁻ < CO
Strong field ligands (CN⁻, CO, NO₂⁻): cause large Δo, electrons pair up → low spin complex
Weak field ligands (halides, H₂O): cause small Δo → electrons remain unpaired → high spin complex
Tetrahedral Field Splitting
In tetrahedral complexes, splitting is reversed: e set (lower energy), t₂ set (higher energy). Δt ≈ (4/9)Δo. All tetrahedral complexes are high spin because Δt is too small for pairing.
6. Isomerism in Coordination Compounds
| Type | Description | Example |
|---|---|---|
| Ionisation isomerism | Same molecular formula but different ions inside/outside coordination sphere | [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br |
| Linkage isomerism | Ambidentate ligand coordinates through different atoms | [Co(NH₃)₅NO₂]²⁺ (nitro-N) and [Co(NH₃)₅ONO]²⁺ (nitrito-O) |
| Geometrical (cis-trans) | Different spatial arrangement of ligands around metal | cis-[PtCl₂(NH₃)₂] and trans-[PtCl₂(NH₃)₂] |
| Optical isomerism | Non-superimposable mirror images (enantiomers) | [Co(en)₃]³⁺ (d and l forms) |
| Coordination isomerism | Both cation and anion are complex ions; ligands are distributed differently | [Co(NH₃)₆][Cr(CN)₆] and [Cr(NH₃)₆][Co(CN)₆] |
7. Important Coordination Compounds
| Compound | Formula | Significance |
|---|---|---|
| Cis-platin | cis-[PtCl₂(NH₃)₂] | Anticancer drug |
| Carboplatin | Second generation Pt complex | Anticancer with fewer side effects |
| Haemoglobin | Fe(II) porphyrin complex | Oxygen transport in blood |
| Chlorophyll | Mg(II) porphyrin complex | Photosynthesis |
| Vitamin B12 | Co(III) complex (cobalamin) | Required for nerve function |
| EDTA complexes | Hexadentate ligand complex | Water softening, analytical chemistry |
JEE Exam Strategy — Coordination Compounds
- Master IUPAC naming rules — 1–2 questions on naming almost every year
- Know hybridisation table for different coordination numbers
- Remember spectrochemical series: weak to strong field order
- VBT: inner (d²sp³, low spin) vs outer (sp³d², high spin) orbital complexes
- CFT: octahedral splitting — t₂g lower, eg higher energy
- Isomerism types with examples: ionisation and geometrical isomers are most tested
- Remember cis-platin (anticancer) formula and trans-platin (inactive)
Practice MCQs — Coordination Compounds
Attempt these 10 JEE Main level MCQs on coordination compounds:
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Frequently Asked Questions (FAQs)
Q1. What is the difference between ionisation isomerism and linkage isomerism?
In ionisation isomerism, the compounds have the same molecular formula but differ in which ligand is inside the coordination sphere and which is outside as a counter ion. For example, [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br give different ions in solution. In linkage isomerism, the same ambidentate ligand is bonded through different donor atoms in the two isomers. For example, in [Co(NH₃)₅NO₂]²⁺, the NO₂⁻ can bond through N (nitro) or O (nitrito) to give two different compounds.
Q2. Why is cis-platin an anticancer drug but trans-platin is not?
Cis-platin [cis-PtCl₂(NH₃)₂] is an anticancer drug because the two Cl⁻ ligands on the same side can cross-link with guanine bases on the same strand of DNA, preventing replication of cancer cells. In trans-platin, the two Cl⁻ are on opposite sides, and they can only cross-link between two different DNA strands with different geometry, which does not effectively inhibit DNA replication. Hence the spatial arrangement matters critically for biological activity.
Q3. What is CFSE and why is it important?
CFSE (Crystal Field Stabilisation Energy) is the extra stability gained by a coordination compound due to the splitting of d-orbitals in a crystal field. When electrons preferentially occupy the lower t₂g set (in octahedral complexes), the system gains stability equal to 0.4Δo per electron in t₂g. CFSE helps explain why certain metal ions form stable complexes, the geometry preference of a complex, and the colours and magnetic properties of coordination compounds.
Q4. How many questions come from Coordination Compounds in JEE Main?
JEE Main typically features 3–4 questions from Coordination Compounds (NCERT Class 12, Chapter 9). The most tested areas are: IUPAC nomenclature, VBT hybridisation and inner/outer orbital complexes, crystal field splitting and high/low spin identification, types of isomerism, and important coordination compounds (cis-platin, haemoglobin). This is a highly rewarding chapter that rewards systematic study of NCERT thoroughly.