Coordination Compounds — Previous Year Questions (Class 12 Chemistry)
Coordination Compounds explores complex ions, ligands, geometry, and crystal field theory. These compounds are crucial in biological systems, medicine, and
TL;DR: Coordination Compounds explores complex ions, ligands, geometry, and crystal field theory. These compounds are crucial in biological systems, medicine…
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Coordination Compounds explores complex ions, ligands, geometry, and crystal field theory. These compounds are crucial in biological systems, medicine, and
Coordination Compounds — Previous Year Questions with Solutions
Q (2023, 2 marks): Define coordination number and coordination sphere. Give one example.
Answer: Coordination number: The number of ligands directly bonded to the central metal atom in a coordination complex.
Coordination sphere: The central metal atom together with the ligands directly bonded to it, written inside square brackets.
Example: [Cu(NH3)4]²⁺
- Central metal atom: Cu²⁺
- Ligands: NH3 (4 molecules)
- Coordination number: 4
- Coordination sphere: [Cu(NH3)4]²⁺
Q (2022, 2 marks): Write the IUPAC name of [CrCl3(H2O)3].
Answer: IUPAC naming rules:
1. Count and name anions/molecules first, then cation
2. Ligands listed alphabetically
3. Cl⁻ = chloro, H2O = aqua
4. Use numerical prefixes (tri-, tetra-, etc.)
5. Indicate charge of complex
[CrCl3(H2O)3]: Ligands are 3 Cl⁻ and 3 H2O
Alphabetically: aqua comes before chloro
IUPAC name: triaquatrichloridochromium(III)
or: triacquatrichloridochromium(III)
Q (2023, 3 marks): Define ligand and classify monodendate, bidentate ligands with examples.
Answer: Ligand: A molecule or ion that can donate electron pair(s) to a central metal atom/ion through coordinate covalent bonds.
Monodentate ligands: Donate one electron pair
Examples: NH3 (ammonia), Cl⁻ (chloride), H2O (water), CN⁻ (cyanide)
Bidentate ligands: Donate two electron pairs through two different donor atoms
Examples:
1. en (ethylenediamine): H2N-CH2-CH2-NH2 (two N atoms)
2. ox (oxalate, C2O4²⁻): O=C-C=O (two O atoms)
3. dien (diethylenetriamine)
Bidentate ligands form more stable complexes than monodentate (chelate effect)
Q (2021, 3 marks): Explain Crystal Field Splitting Theory (CFST) for octahedral geometry.
Answer: Crystal Field Splitting Theory for octahedral complexes:
1. In isolated atom: d-orbitals have same energy (degenerate)
2. In octahedral field: Ligands (negative charges) repel electrons in d-orbitals
3. Splitting pattern:
- dz² and dx²-y² point toward ligands: HIGH ENERGY (eg)
- dxy, dxz, dyz point between ligands: LOW ENERGY (t2g)
- Energy gap = Δ0 (crystal field splitting parameter)
4. Effect of strong vs weak field ligands:
- Strong field (CN⁻, CO): Large Δ0, low-spin (paired electrons)
- Weak field (I⁻, H2O): Small Δ0, high-spin (unpaired electrons)
5. Pairing energy vs splitting energy determines electron configuration
Q (2022, 3 marks): How many isomers are possible for [Co(NH3)4Cl2]⁺? Name them.
Answer: For [Co(NH3)4Cl2]⁺ in octahedral geometry:
Two Cl⁻ ligands can occupy different positions
1. Cis-isomer (1,2-positions): Cl atoms adjacent
IUPAC name: dichlorotetramminecobalt(III)
Color: Purple/violet (due to specific d-d transitions)
2. Trans-isomer (1,4-positions/opposite): Cl atoms across
IUPAC name: trans-dichlorotetramminecobalt(III)
Color: Green (different electronic transitions)
Total isomers = 2 (geometric isomerism)
Note: These are also optical isomers if asymmetric; however, the main isomerism here is geometric
Q (2023, 2 marks): Define chelation and explain why chelate complexes are more stable.
Answer: Chelation: A process in which a polydentate ligand forms multiple coordinate bonds with a single metal ion, creating a ring structure (chelate ring).
Why chelate complexes are more stable (Chelate Effect):
1. Entropy factor: Monodentate displacement requires breaking/forming multiple bonds with release of free ligand molecules, increasing entropy (unfavorable)
2. Multiple bonds: Bidentate/polydentate ligands form multiple coordinate bonds with same metal, making them harder to displace
3. Ring stability: Once formed, ring closure is kinetically favorable
4. Thermodynamic stability: ΔG is more negative for chelate formation
Example: [Cu(en)3]²⁺ (with ethylenediamine) is more stable than [Cu(NH3)6]²⁺
Frequently Asked Questions
What is the difference between coordination number and oxidation state?
Coordination number is the number of ligand atoms bonded to the central metal atom (spatial/geometric). Oxidation state is the charge of the central metal atom after removing all ligands (electron accounting). They are independent properties.
How do you determine the geometry of a complex?
Coordination number determines geometry: CN = 4 (tetrahedral/square planar), CN = 6 (octahedral), CN = 2 (linear). For d⁸ metals like Ni²⁺ and Pt²⁺, CN = 4 gives square planar. Crystal field splitting and ligand field strength influence preferred geometry.
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