Chemistry 2024 Paper I 50 marks Elucidate

Q7

(a) Show the splitting of 'd' orbitals in square planar field according to Crystal Field Theory (CFT). Comment on the following statement : 'The difference in energy between the dₓ²₋ᵧ² and dₓᵧ orbitals in square planar field is identical to the difference between the same orbitals in the octahedral field.' (10 marks) (b) (i) The bond orders of some metal carbonyls are : M – C bond order | C – O bond order Ni(CO)₄ | 1·33 | 2·64 [Co(CO)₄]⁻ | 1·89 | 2·14 [Fe(CO)₄]²⁻ | 2·16 | 1·85 Explain the above facts. (10 marks) (ii) Identify A, B and C. What is the relationship between A and B ? [Co(NH₃)₅Cl]Cl₂ → A (NaNO₂) A ⇄ B (Let stand or warm, HCl / UV ray) [Co(NH₃)₅Cl]Cl₂ → C (Dilute NH₃ aqueous) C → B (NaNO₂, concentrated HCl) (13 marks) (c) (i) On the basis of Crystal Field Theory, account for the following statement : While [CoF₆]³⁻ is paramagnetic, [Co(CN)₆]³⁻ is diamagnetic. (10 marks) (ii) Elucidate the structure(s) of Co₂(CO)₈. Comment on its magnetic behaviour. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) क्रिस्टल क्षेत्र सिद्धांत (CFT) के अनुसार वर्ग समतली क्षेत्र में 'd' कक्षकों के विपाटन को प्रदर्शित कीजिए। निम्नलिखित कथन पर टिप्पणी कीजिए : 'वर्ग समतली क्षेत्र में dₓ²₋ᵧ² और dₓᵧ कक्षकों के मध्य ऊर्जा का अंतर, अष्टफलकीय क्षेत्र में समान कक्षकों के मध्य अंतर के समरूप है।' (10 अंक) (b) (i) कुछ धातु कार्बोनिलों के आबंध क्रम हैं : M – C आबंध क्रम | C – O आबंध क्रम Ni(CO)₄ | 1·33 | 2·64 [Co(CO)₄]⁻ | 1·89 | 2·14 [Fe(CO)₄]²⁻ | 2·16 | 1·85 उपर्युक्त तथ्यों की व्याख्या कीजिए। (10 अंक) (ii) A, B और C को पहचानिए। A और B के बीच में क्या संबंध है ? [Co(NH₃)₅Cl]Cl₂ → A (NaNO₂) A ⇄ B (शांत छोड़ें या गर्म, HCl / UV किरण) [Co(NH₃)₅Cl]Cl₂ → C (तनु NH₃ जलीय) C → B (NaNO₂, सांद्र HCl) (13 अंक) (c) (i) क्रिस्टल क्षेत्र सिद्धांत के आधार पर, निम्नलिखित कथन का विवरण दीजिए : जबकि [CoF₆]³⁻ अनुचुंबकीय है, [Co(CN)₆]³⁻ प्रतिचुंबकीय है । (10 अंक) (ii) Co₂(CO)₈ की संरचना/संरचनाओं को स्पष्ट कीजिए । इसके चुंबकीय आचरण पर टिप्पणी कीजिए । (15 अंक)

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Directive word: Elucidate

This question asks you to elucidate. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

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How this answer will be evaluated

Approach

Elucidate the crystal field splitting patterns, metal-ligand bonding in carbonyls, and coordination compound reactions systematically. Allocate approximately 25% time to part (a) on square planar splitting, 35% to part (b) covering carbonyl bond orders and linkage isomerism, and 40% to part (c) on magnetic properties and cluster structures. Structure the answer with clear sub-headings, diagrams for each splitting/structure, and concluding remarks on the relationship between electronic structure and properties.

Key points expected

  • Part (a): Correct d-orbital splitting diagram for square planar field (D4h) showing dx²-y² > dxy > dz² > dxz/dyz order; explicit comparison with octahedral splitting showing Δsp = 1.456Δo and that dxy-dx²-y² gap equals Δo, not identical to octahedral where they are degenerate
  • Part (b)(i): Explanation using π-backbonding and synergic effect—higher negative charge on metal increases M→CO π-back-donation, raising M-C bond order and lowering C-O bond order; correlation with 18-electron rule and IR spectroscopic evidence
  • Part (b)(ii): Identification: A = [Co(NH₃)₅NO₂]Cl₂ (nitro), B = [Co(NH₃)₅ONO]Cl₂ (nitrito), C = [Co(NH₃)₅OH]Cl₂; A and B are linkage isomers (ambidentate NO₂⁻ coordination)
  • Part (c)(i): CFT explanation—F⁻ is weak field (Δo < P), t2g⁴eg² configuration, 4 unpaired electrons (paramagnetic); CN⁻ is strong field (Δo > P), t2g⁶eg⁰, low-spin diamagnetic; explicit crystal field stabilization energy calculation
  • Part (c)(ii): Co₂(CO)₈ exists as bridged (C2v, two μ-CO) and non-bridged (D3d, Co-Co bond) isomers in equilibrium; bridged form has no unpaired electrons (diamagnetic), non-bridged has one unpaired electron per Co (paramagnetic); structural drawings showing 18-electron count

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%14Demonstrates precise understanding of CFT energy level ordering in square planar vs octahedral fields; correctly applies π-backbonding theory to explain inverse correlation of M-C and C-O bond orders; accurately identifies linkage isomerism and distinguishes strong/weak field effects on spin states; correctly describes Co₂(CO)₈ isomerism and electron countingShows basic understanding of splitting patterns and bond order trends but confuses energy gap relationships or misidentifies isomers; partial explanation of magnetic differences without explicit Δo vs P comparison; vague description of carbonyl structuresFundamental errors in splitting order (e.g., placing dz² highest), incorrect identification of A/B/C, confuses paramagnetic/diamagnetic nature, or describes Co₂(CO)₈ as single structure without isomerism
Mechanism / equation20%14Provides complete reaction mechanisms: acid-base equilibrium for C formation, nucleophilic attack pathway for nitro→nitrito isomerization; writes balanced equations for all transformations; shows electron flow in π-backbonding with orbital diagrams; explains kinetic vs thermodynamic control in linkage isomerismWrites most equations correctly but misses mechanistic details of isomerization; describes backbonding qualitatively without orbital interaction diagrams; incomplete explanation of reaction conditions (warm, HCl, UV) effectsMissing or incorrect chemical equations; no mechanistic insight for isomerization; fails to explain role of reagents (NaNO₂, HCl, NH₃) in directing products
Numerical accuracy20%14Correctly calculates Δsp = 1.456Δo relationship; performs CFSE calculations for both high-spin and low-spin Co(III) complexes showing stabilization differences; verifies 18-electron rule for all carbonyl species; uses bond order data quantitatively to support backbonding argument with proportional reasoningStates correct relationships without derivation; attempts CFSE calculations with minor errors; mentions 18-electron rule without explicit electron counting; qualitative use of bond order dataIncorrect numerical relationships (e.g., Δsp = Δo); no CFSE calculations; wrong electron counts for carbonyls; ignores quantitative bond order data entirely
Diagram / structure20%13Clear labeled diagrams: square planar splitting with correct relative energies and electron filling; octahedral splitting for comparison; orbital overlap diagrams for M-CO π-backbonding; structural drawings of both Co₂(CO)₈ isomers showing bridging vs terminal CO; crystal field splitting diagrams for [CoF₆]³⁻ and [Co(CN)₆]³⁻ with electron configurationsMost diagrams present but lacking labels or with minor energy level errors; incomplete structural representations; splitting diagrams without electron filling shownMissing critical diagrams (especially square planar splitting or Co₂(CO)₈ structures); poorly drawn or unlabeled diagrams; incorrect orbital shapes or energy ordering
Application context20%13Connects square planar splitting to trans effect in Pt(II) chemistry (relevant to Indian pharmaceutical cisplatin research); relates carbonyl bond orders to IR spectroscopy (νCO) and homogeneous catalysis (Monsanto acetic acid process); discusses linkage isomerism relevance to cancer therapy (nitroprusside); explains Co₂(CO)₈ as hydroformylation catalyst precursorMentions spectroscopic correlations (IR/Raman) without specific examples; general reference to catalysis without named processes; limited connection to real-world applicationsNo application context provided; fails to relate theoretical concepts to experimental techniques or industrial processes; isolated presentation of facts

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