Science Class 12 - Coordination Compounds Notes

Coordination Compounds

Welcome to the chapter on Coordination Compounds for Class 12. In this chapter, you will learn about the structure, bonding, nomenclature, and properties of coordination compounds. By the end of this chapter, you will understand their importance in chemistry and their applications in real life.

Introduction

Coordination compounds are compounds in which a central metal atom or ion is surrounded by a set of ligands. These compounds play a vital role in biological systems, industrial processes, and analytical chemistry.

Key Concepts

• Coordination Compound: A compound consisting of a central metal atom/ion bonded to ligands.
• Ligand: An ion or molecule that donates a pair of electrons to the metal atom/ion.
• Coordination Number: The number of ligand donor atoms bonded to the central metal atom/ion.
• Coordination Sphere: The central atom/ion and the ligands attached to it, usually shown in square brackets.

Nomenclature of Coordination Compounds

• Name the ligands first (in alphabetical order), then the central metal atom/ion.
• Use prefixes (di-, tri-, tetra-, etc.) to indicate the number of each type of ligand.
• If the complex ion is an anion, the metal name ends with -ate.
• Oxidation state of the metal is shown in Roman numerals in parentheses.

Example: [Co(NH₃)₆]Cl₃ is named as Hexaamminecobalt(III) chloride.

Types of Ligands

• Monodentate: Ligands that bind through one donor atom (e.g., Cl^-, NH₃).
• Bidentate: Ligands that bind through two donor atoms (e.g., ethylenediamine, C₂O₄^2-).
• Polydentate: Ligands that bind through more than two donor atoms (e.g., EDTA).

Bonding in Coordination Compounds

• Werner’s Theory: Explains primary and secondary valencies in complexes.
• Valence Bond Theory (VBT): Explains the geometry and magnetic properties of complexes.
• Crystal Field Theory (CFT): Explains color, magnetism, and stability of complexes based on d-orbital splitting.

Isomerism in Coordination Compounds

• Structural Isomerism: Linkage, ionisation, coordination, and hydrate isomerism.
• Stereoisomerism: Geometrical and optical isomerism.

Applications of Coordination Compounds

• Biological systems (e.g., hemoglobin, chlorophyll).
• Medicines (e.g., cisplatin for cancer treatment).
• Analytical chemistry (e.g., detection of metal ions).
• Industrial processes (e.g., extraction and purification of metals).

Practice Questions

1. Name the following compound: [Fe(CN)₆]^4-
2. What is the coordination number of [Cr(H₂O)₆]³⁺?
3. Give one example each of a monodentate and a bidentate ligand.
4. What is geometrical isomerism? Give an example.
5. Explain the difference between primary and secondary valency.

Challenge Yourself

• Draw the structure of [Ni(CO)₄] and state its geometry.
• Explain the color of [Ti(H₂O)₆]³⁺ using crystal field theory.

Did You Know?

• Alfred Werner won the Nobel Prize in Chemistry in 1913 for his work on coordination compounds.
• Vitamin B₁₂ is a coordination compound essential for our health.

Glossary

• Ligand: An ion or molecule that binds to a central metal atom/ion.
• Coordination Number: Number of ligand donor atoms attached to the central atom/ion.
• Isomerism: Existence of compounds with the same formula but different structures or arrangements.
• Crystal Field Splitting: The separation of d-orbitals in a metal complex due to the presence of ligands.

Answers to Practice Questions

1. Hexacyanoferrate(II) ion
2. 6
3. Monodentate: Cl^-; Bidentate: Ethylenediamine (en)
4. Geometrical isomerism is the existence of different spatial arrangements of ligands (e.g., cis- and trans- forms in [Pt(NH₃)₂Cl₂]).
5. Primary valency is the oxidation state of the metal; secondary valency is the number of ligands attached to the metal.

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Coordination compounds are everywhere—from your blood to medicines! Explore their world to understand chemistry better.