These extra practice questions for Class 12 Chemistry Chapter 5 – Coordination Compounds go beyond the NCERT textbook exercises to reinforce nomenclature, isomerism, bonding theories, and the applications of coordination compounds. Useful for board exam revision and quick concept checks.
Very Short Answer Type Questions (1 Mark)
Q1. Define coordination number of a metal ion in a complex.
Ans: The coordination number is the total number of ligand donor atoms directly bonded to the central metal ion in a coordination entity, e.g. 6 in [Co(NH3)6]3+.
Q2. What type of isomerism is exhibited by [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2?
Ans: Linkage isomerism — the ambidentate ligand NO2− is bonded through nitrogen (nitrito-N) in the first and through oxygen (nitrito-O) in the second.
Q3. Name the theory that explains the colour and magnetic properties of coordination compounds in terms of d-orbital splitting.
Ans: Crystal Field Theory (CFT).
Q4. Give an example of an ambidentate ligand and its two modes of linkage.
Ans: SCN− — it can bind through sulphur (thiocyanato-S) or through nitrogen (isothiocyanato-N).
Short Answer Type Questions (2–3 Marks)
Q5. Why is [Ni(CN)4]2− square planar and diamagnetic, while [NiCl4]2− is tetrahedral and paramagnetic, even though both contain Ni2+?
Ans: Ni2+ is 3d8 in both. CN− is a strong-field ligand, so it forces the two unpaired 3d electrons to pair up, freeing a d-orbital for dsp2 hybridisation (square planar, diamagnetic). Cl− is a weak-field ligand, so pairing does not occur and the complex instead adopts sp3 hybridisation (tetrahedral), retaining 2 unpaired electrons and paramagnetism.
Q6. Calculate the spin-only magnetic moment of [CoF6]3−.
Ans: Co3+ is 3d6; F− is a weak-field ligand, giving a high-spin configuration (t2g4eg2) with n=4 unpaired electrons. μ=√n(n+2) BM=√(4×6)=√24≈4.90 BM.
Q7. What is the difference between homoleptic and heteroleptic complexes? Classify [Co(NH3)4Cl2]+ and [Cr(NH3)6]3+.
Ans: A homoleptic complex has only one kind of ligand bonded to the metal; a heteroleptic complex has more than one kind. [Co(NH3)4Cl2]+ is heteroleptic (NH3 and Cl−); [Cr(NH3)6]3+ is homoleptic (only NH3).
Higher Order Thinking Skills (HOTS)
Q8. Two octahedral complexes of Co3+, one with F− ligands and one with NH3 ligands, show very different magnetic behaviour despite having the same metal and oxidation state. Explain what this tells you about crystal field splitting, and predict which complex is paramagnetic.
Ans: Co3+ is 3d6 in both complexes. Since the two complexes behave differently, this shows that the ligand’s field strength, not just the metal ion, determines whether the complex is high spin or low spin. F− is a weak-field ligand (early in the spectrochemical series), so [CoF6]3− is high spin (t2g4eg2, 4 unpaired electrons) and paramagnetic. NH3 is a comparatively strong-field ligand, so [Co(NH3)6]3+ is low spin (t2g6eg0, 0 unpaired electrons) and diamagnetic.
Q9. Explain, with reference to the chelate effect, why EDTA forms an exceptionally stable complex with metal ions like Ca2+, and why this makes it useful both in water-hardness titrations and in treating heavy-metal poisoning.
Ans: EDTA is a hexadentate ligand (six donor atoms: four carboxylate oxygens and two amine nitrogens), so it wraps around a metal ion and forms five interlocking five-membered chelate rings in a single complex. This multiple-ring “chelate effect” makes the [M(EDTA)]n− complex far more thermodynamically stable than complexes of the same metal with several separate unidentate ligands, since forming one hexadentate complex loses far less entropy than forming six separate metal-ligand bonds independently. This exceptional stability is exactly why EDTA is used to selectively and tightly bind Ca2+/Mg2+ in water-hardness titrations (the endpoint reflects essentially complete complexation), and why it can bind toxic heavy-metal ions (e.g. Pb2+) tightly enough in the body to allow their safe excretion during chelation therapy.
Continue Practising — NCERT Solutions for Class 12 Chemistry:
Chapter 1: Solutions | Chapter 2: Electrochemistry | Chapter 3: Chemical Kinetics | Chapter 4: The d- and f-Block Elements | Chapter 5: Coordination Compounds
