Class 12 Chemistry Chapter 9 Notes: Coordination Compounds | Important Questions & PYQs

 Class 12 Chemistry – Chapter 9 Coordination Compounds Notes

1. What are Coordination Compounds?

• Compounds in which a central metal atom/ion is bonded to surrounding ligands by coordinate (dative) bonds.

• Example:

  $$[Cu(N{H}_3{)}_4{]}^{2+},[Fe(CN{)}_6{]}^{3-}$$

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2. Werner’s Theory of Coordination Compounds

Main Postulates:

• Metals show two types of valencies:

  • Primary valency → Ionisable (oxidation state)

  • Secondary valency → Non-ionisable (coordination number)

• Secondary valencies are fixed in number and directed in space

• Geometry depends on number of secondary valencies

📌 Example:
CoCl₃·6NH₃

• Primary valency = 3 (Cl⁻)

• Secondary valency = 6 (NH₃) → octahedral

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3. Important Terminology

(A) Coordination Entity

• Central metal + ligands in square brackets

• Example: [Co(NH3)6]³⁺

(B) Ligands

• Molecules/ions donating lone pair to metal

Types of ligands:

• Unidentate: NH₃, Cl⁻

• Bidentate: en, C₂O₄²⁻

• Polydentate: EDTA

• Ambidentate: NO₂⁻, SCN⁻

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(C) Coordination Number

• Number of donor atoms attached to metal

• Example: [Fe(CN)6]³⁻ → CN = 6

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(D) Coordination Polyhedron

• Spatial arrangement of ligands

• CN = 6 → Octahedral

• CN = 4 → Tetrahedral / Square planar

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(E) Homoleptic & Heteroleptic

• Homoleptic → Same ligands only
  [Ni(CO)4]

• Heteroleptic → Different ligands
  [Co(NH3)4Cl2]⁺

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4. Nomenclature of Coordination Compounds (IUPAC)

Basic Rules

1. Name cation first, then anion

2. Ligands named alphabetically

3. Negative ligands end with –ido

4. Neutral ligands:

   • NH₃ → ammine

   • H₂O → aqua

   • CO → carbonyl

5. Oxidation state in Roman numerals

📌 Example:
[Co(NH3)5Cl]Cl2
→ Pentaamminechloridocobalt(III) chloride

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5. Isomerism in Coordination Compounds

(A) Structural Isomerism

• Ionisation isomerism

• Linkage isomerism

• Coordination isomerism

(B) Stereoisomerism

• Geometrical isomerism

  • cis–trans

  • fac–mer

• Optical isomerism

  • Non-superimposable mirror images

📌 Example:

• [Co(en)3]³⁺ → optical isomerism

• [Pt(NH3)2Cl2] → cis/trans

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6. Bonding in Coordination Compounds

(A) Valence Bond Theory (VBT)

• Explains:

  • Geometry

  • Magnetic nature

• Inner orbital complex → strong field ligand

• Outer orbital complex → weak field ligand

📌 Example:

• [Ni(CN)4]²⁻ → square planar, diamagnetic

• [NiCl4]²⁻ → tetrahedral, paramagnetic

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(B) Crystal Field Theory (CFT)

Octahedral Splitting

• d-orbitals split into:

  • t₂g (lower energy)

  • e₉ (higher energy)

High Spin vs Low Spin

• Weak field ligand (Δ₀ < P) → high spin

• Strong field ligand (Δ₀ > P) → low spin

📌 Example:

• [Fe(H2O)6]³⁺ → high spin

• [Fe(CN)6]³⁻ → low spin

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7. Spectrochemical Series

Increasing ligand field strength:

$$I^{-}<B{r}^{-}<C{l}^{-}<F^{-}<O{H}^{-}<H_{2}O<N{H}_3<en<N{O}_2^{-}<C{N}^{-}<CO$$

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8. Magnetic Properties

• Depends on number of unpaired electrons

• Spin-only formula:

$$\mu =\sqrt{n(n+2)}\text{ BM}$$

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9. Stability of Coordination Compounds

Stability Constant (β)

• Higher β → more stable complex

Factors Affecting Stability

• Nature of metal ion

• Oxidation state

• Nature of ligand

• Chelate effect

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10. Chelate Effect

• Complexes with polydentate ligands are more stable

• Due to formation of ring structures

📌 Example:

• [Cu(en)2]²⁺ more stable than [Cu(NH3)4]²⁺

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11. Bonding in Metal Carbonyls

• M–CO bond has:

  • σ-bond (CO → metal)

  • π-back bonding (metal → CO)

📌 Example:

• [Ni(CO)4] → diamagnetic

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12. Colour in Coordination Compounds

• Due to d–d transitions

• Different ligands → different Δ₀ → different colour

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13. Importance & Applications

(A) Biological

• Haemoglobin (Fe)

• Chlorophyll (Mg)

• Vitamin B₁₂ (Co)

(B) Medicinal

• EDTA → lead poisoning

• Cis-platin → cancer treatment

(C) Analytical

• EDTA titrations

• DMG test for Ni²⁺

(D) Metallurgy

• Gold & silver extraction using CN⁻ complexes

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14. Exam Super-Important Points

✔ Square planar → mostly diamagnetic
✔ CN⁻, CO → strong field ligands
✔ Chelate complexes → most stable
✔ Colour depends on ligand, not only metal
✔ [Ni(CO)4] → oxidation state = 0

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are coordination compounds?
   Compounds in which a central metal atom/ion is bonded to ligands by coordinate (dative) bonds.

2. What is a coordination entity?
   Central metal atom/ion along with ligands enclosed in square brackets.

3. Define ligand.
   A ligand is an ion or molecule that donates a lone pair of electrons to the central metal atom.

4. What is coordination number?
   Number of donor atoms directly attached to the central metal atom.

5. What are ambidentate ligands?
   Ligands that can coordinate through two different atoms.
   Example: NO₂⁻, SCN⁻

6. What is meant by homoleptic complex?
   A complex containing only one type of ligand.

7. What type of isomerism is shown by [Pt(NH₃)₂Cl₂]?
   Geometrical (cis–trans) isomerism.

8. Why are some coordination compounds coloured?
   Due to d–d electronic transitions.

9. What is chelate effect?
   Higher stability of complexes formed by polydentate ligands.

10. Why is [Ni(CO)₄] diamagnetic?
    Because it has no unpaired electrons.

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📝 Long Answer Questions

1. Explain Werner’s theory of coordination compounds.

Werner proposed that metals show two types of valencies:

• Primary valency

  • Ionisable

  • Corresponds to oxidation state

• Secondary valency

  • Non-ionisable

  • Corresponds to coordination number

  • Fixed and directed in space

Example: CoCl₃·6NH₃

• Primary valency = 3 (Cl⁻)

• Secondary valency = 6 (NH₃)

• Geometry = octahedral

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2. Explain types of ligands with examples.

Ligands are classified based on number of donor atoms:

• Unidentate: Donate one pair
  Example: NH₃, Cl⁻

• Bidentate: Donate two pairs
  Example: en, C₂O₄²⁻

• Polydentate: Donate more than two pairs
  Example: EDTA

• Ambidentate: Coordinate through two different atoms
  Example: NO₂⁻, SCN⁻

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3. Explain crystal field theory (CFT) for octahedral complexes.

In octahedral complexes:

• Five d-orbitals split into two sets:

  • t₂g (lower energy): dxy, dxz, dyz

  • eg (higher energy): dx²–y², dz²

High spin complex

• Weak field ligand

• Δ₀ < pairing energy

Low spin complex

• Strong field ligand

• Δ₀ > pairing energy

Examples:

• [Fe(H₂O)₆]³⁺ → high spin

• [Fe(CN)₆]³⁻ → low spin

📝 PYQs (Previous Years’ Questions – CBSE Board Oriented)

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🔹 1 Mark PYQs

1. What is a coordination compound?

2. Define coordination number.

3. What is a ligand?

4. Name one ambidentate ligand.

5. What is the oxidation state of Ni in [Ni(CO)₄]?

6. Which type of bond is present between metal and ligand?

7. Why is [Ni(CO)₄] diamagnetic?

8. Write the IUPAC name of NH₃ ligand.

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🔹 2 Mark PYQs

1. Differentiate between double salt and coordination compound.

2. What is chelate effect?

3. What are homoleptic and heteroleptic complexes?

4. Write two postulates of Werner’s theory.

5. What is linkage isomerism? Give one example.

6. Write two differences between strong field and weak field ligands.

7. Why do coordination compounds show colour?

8. Write two applications of coordination compounds.

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🔹 3 Mark PYQs

1. Explain Werner’s theory with suitable example.

2. Explain types of ligands on the basis of denticity.

3. Explain geometrical isomerism with example.

4. Describe crystal field splitting in octahedral complexes.

5. Explain magnetic behaviour of coordination compounds.

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🔹 5 Mark PYQs

1. (a) What are coordination compounds?
   (b) Explain Werner’s theory of coordination compounds.

2. (a) Explain isomerism in coordination compounds.
   (b) Describe any two types with examples.

3. (a) Explain valence bond theory for coordination compounds.
   (b) Predict geometry and magnetic behaviour of
   [Ni(CN)₄]²⁻ and [NiCl₄]²⁻.

4. (a) Explain crystal field theory.
   (b) Distinguish between high spin and low spin complexes.

5. (a) What is chelate effect?
   (b) Explain factors affecting stability of coordination compounds.

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🔑 Most Repeated PYQ Areas (Exam Favourite)

✔ Werner’s theory
✔ Types of ligands
✔ Isomerism (especially geometrical & linkage)
✔ VBT vs CFT
✔ High spin vs low spin complexes
✔ Chelate effect & stability constant
✔ [Ni(CN)₄]²⁻ vs [NiCl₄]²⁻ (very common)

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❓ FAQs (Concept Clarity)

1. Why do coordination compounds show isomerism?
   Because of different arrangements of ligands around the central metal.

2. Why are chelate complexes more stable?
   Due to formation of ring structures and chelate effect.

3. Why does colour depend on ligand?
   Different ligands cause different crystal field splitting (Δ₀).

4. Why are square planar complexes usually diamagnetic?
   Because electrons are paired due to strong field ligands.

5. Why is oxidation state of Ni in [Ni(CO)₄] zero?
   Because CO is a neutral ligand.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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