Q.88 Chemistry ⭐ 360° Option Elimination Active
NEET 2023 Chemistry Question 88 Solution
Topic: Coordination Compounds · Official NTA Booklet Code G5
Question
Which complex compound is most stable?
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Verified Against NTA Final Answer Key
Option (4) : [CoCl₂(en)₂]NO₃
📚 Coordination Compounds Easy
📋 Given Parameters & Problem Setup
Complex 1 [Co(NH₃)₆]₂(SO₄)₃
Complex 2 [Co(NH₃)₄(H₂O)Br](NO₃)₂
Complex 3 [Co(NH₃)₃(NO₃)₃]
Complex 4 [CoCl₂(en)₂]NO₃
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⏱️ Save ~25 seconds The 30-Second Exam Hall Shortcut: ⚡ Chelate Effect Dominance: Chelating Ligand (en) Imparts Highest Stability
Chelate complexes containing multidentate ring-forming ligands are far more stable than non-chelated complexes containing only monodentate ligands. Ethylenediamine (en) is a bidentate chelating ligand forming stable 5-membered rings. Among all four choices, ONLY Option (4) contains chelating ligands!
1 Scan ligand types in all four complexes:
2 - Complex (1): NH₃ is unidentate (monodentate).
3 - Complex (2): NH₃, H₂O, Br⁻ are all unidentate.
4 - Complex (3): NH₃, NO₃⁻ are unidentate.
5 - Complex (4): "en" (ethylenediamine) is a bidentate chelating ligand.
6 Chelate rings provide huge positive entropy gain (chelate effect), maximizing formation constant β.
7 Select Option (4) [CoCl₂(en)₂]NO₃ instantly!
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Why You Can 100% Trust This Shortcut: Classical Coordination Chemistry Chelate Effect in NCERT.
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Define the Chelate Effect in Coordination Chemistry
When a multidentate (polydentate) ligand binds to a central metal ion to form a ring structure, the resulting complex possesses significantly higher thermodynamic stability than an analogous complex containing only unidentate ligands. This phenomenon is known as the chelate effect:
Step 2
Analyze Ligand Denticity in Given Complexes
Examine each coordination sphere: [Co(NH₃)₆]³⁺, [Co(NH₃)₄(H₂O)Br]²⁺, and [Co(NH₃)₃(NO₃)₃] contain only unidentate ligands. [CoCl₂(en)₂]⁺ contains two bidentate chelating ethylenediamine ligands:
Step 3
Thermodynamic Driving Force for Chelation
Chelate formation is thermodynamically favored primarily due to a positive entropy change (ΔS° > 0). Displacement of multiple monodentate solvent molecules by a chelating ligand increases disorder:
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 (1)
✗ Incorrect Verdict & Scientific Reason:
Contains only monodentate NH₃ ligands with no chelate rings.
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Student Fallacy (Why candidates pick this trap): Candidate incorrectly thought a higher number of ammine ligands implies higher stability.
2 (2)
✗ Incorrect Verdict & Scientific Reason:
Contains weak and monodentate ligands (H₂O, Br⁻) with no chelation.
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Student Fallacy (Why candidates pick this trap): Candidate confused mixed-ligand sphere with thermodynamic stability.
3 (3)
✗ Incorrect Verdict & Scientific Reason:
Neutral complex without chelate rings, possessing significantly lower formation constant.
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Student Fallacy (Why candidates pick this trap): Candidate thought zero charge on the complex sphere provides greater stability.
4 (4)
✓ 100% Correct Verdict & Scientific Reason:
Accurate: Contains two bidentate chelating ethylenediamine ligands forming stable 5-membered rings.
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Why this is airtight: None. Candidate recognized that chelation dominates complex stability.
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NTA Trap Alert: Overlooking the Chelate Effect in Favor of Ligand Field Strength
Students often try to compare ligand field strengths in the spectrochemical series (e.g., NH₃ vs Cl⁻). However, thermodynamic stability of a coordination complex is overwhelmingly governed by the chelate effect: a complex with chelate rings is vastly more stable than non-chelated complexes even if individual donor atoms are comparable.
🛡️ How to avoid the -1 mark: Whenever asked for "most stable complex", immediately look for multidentate/chelating ligands (en, ox, EDTA).
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Core NEET Syllabus Standard
National Eligibility cum Entrance Test (NEET UG) · Coordination Compounds Standard Benchmark
Core Scientific Principle Tested:
"Polydentate ligands form stable ring structures with metal ions, dramatically enhancing the formation constant (chelate effect)."
Syllabus Focus: Stability of Coordination Compounds – Chelate Effect
Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗
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