Chemistry 2022 Paper I 50 marks Explain

Q6

(a) (i) Calculate the number of metal-metal bond in Cp₂Fe₂(CO)₄. (ii) Determine the structural type of the metal atom cluster, Bi₅³⁺. (10 marks) (b) Why are 4f metal ions (lanthanide ions) generally pale in colour? Why do they show line like electronic spectra? (10 marks) (c) Consider the above pairs of π-donor ligands: Identify the d⁶ metal ions/atom among Co(O), Mn(I), Fe(II), and Fe(III), which form neutral mixed sandwich compound. Explain. (10 marks) (d) (i) How XeF₆ can be separated from a mixture of XeF₂, XeF₄ and XeF₆? (ii) Write the products with explanation: B₂H₆ + 2NH₃ → ? and B₂H₆ + 2Me₃N → ? (10 marks) (e) Aqueous solution of FeCl₃ is bright yellow and not pale-violet like other metal ions having high-spin d⁵ configuration. Discuss the origin of the colour. (10 marks)

हिंदी में प्रश्न पढ़ें

(a) (i) Cp₂Fe₂(CO)₄ में धातु-धातु आबंधों की संख्या का परिकलन कीजिए। (ii) धातु परमाणु गुच्छ, Bi₅³⁺, के संरचनात्मक प्रकार को निर्धारित करें। (10 अंक) (b) 4f धातु आयनों (लैन्थेनाइड आयनों) का रंग सामान्यतः फीका क्यों होता है? यह रेखा (लाइन) जैसा इलेक्ट्रॉनिक स्पेक्ट्रा क्यों दिखाते हैं? (10 अंक) (c) निम्नलिखित π-दाता संतुलियों (लिगंडों) के जोड़ों को ध्यान में रखें: Co(O), Mn(I), Fe(II), और Fe(III), में से d⁶-धातु आयनों/परमाणुओं को पहचानें जो अनवेशी मिश्रित अंतर्द्वितीय यौगिक बनाते हैं। व्याख्या कीजिए। (10 अंक) (d) (i) XeF₆ को XeF₂, XeF₄ और XeF₆ के मिश्रण से कैसे पृथक किया जा सकता है? (ii) व्याख्या के साथ उत्पादों को लिखें — B₂H₆ + 2NH₃ → ? और B₂H₆ + 2Me₃N → ? (10 अंक) (e) FeCl₃ का जलीय विलयन चमकीला पीला है और यह दूसरे d⁵-उच्च प्रचक्रण विच्यासों के धातु आयनों जैसा फीका बैंगनी नहीं होता है। रंग की उत्पत्ति का वर्णन कीजिए। (10 अंक)

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain requires logical reasoning with supporting evidence across all six sub-parts. Allocate approximately 15% time to (a)(i)-(ii) on metal-metal bonding and Wade-Mingos rules; 20% to (b) on lanthanide spectroscopy; 20% to (c) on 18-electron rule and sandwich compounds; 25% to (d) on noble gas separation and borane chemistry; and 20% to (e) on charge transfer spectra. Structure with brief introductions per sub-part, core explanatory analysis, and concluding synthesis where relevant.

Key points expected

  • (a)(i) Calculation of metal-metal bonds in Cp₂Fe₂(CO)₄ using 18-electron rule: each Fe has 17 electrons in monomeric form, dimer requires 1 M-M bond to satisfy 18-electron configuration
  • (a)(ii) Bi₅³⁺ cluster structure: n+1 = 5 skeletal electron pairs, closo-trigonal bipyramidal geometry per Wade-Mingos rules
  • (b) Lanthanide pale color explanation: 4f orbitals shielded by 5s²5p⁶, weak f-f transitions; line-like spectra due to weak crystal field effects and minimal orbital-lattice coupling
  • (c) d⁶ identification for neutral mixed sandwich: Fe(II) forms ferrocene analogues; apply 18-electron rule to eliminate Co(0), Mn(I), Fe(III) as they yield incorrect electron counts
  • (d)(i) XeF₆ separation via differential reactivity with NaF or thermal gradient; XeF₂ and XeF₄ form adducts or have different volatility
  • (d)(ii) B₂H₆ + 2NH₃ → [H₂B(NH₃)₂]⁺[BH₄]⁻ (ionic adduct) vs B₂H₆ + 2Me₃N → 2Me₃N·BH₃ (symmetrical cleavage); explain based on steric and electronic factors
  • (e) FeCl₃ yellow color: ligand-to-metal charge transfer (LMCT) from Cl⁻ to Fe³⁺ dominates over weak d-d transitions; discuss hydrolysis to [Fe(H₂O)₅OH]²⁺ and colloidal FeO(OH)

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates flawless application of 18-electron rule across (a), (c), (e); correctly identifies Wade-Mingos framework for (a)(ii); explains shielding effects in lanthanides for (b); distinguishes ionic vs symmetrical cleavage in (d)(ii); attributes FeCl₃ color to LMCT not d-d transitionsApplies 18-electron rule with minor errors; identifies closo/nido/closo incorrectly or vaguely; mentions shielding for lanthanides without quantitative reasoning; confuses cleavage modes in borane reactions; attributes color to hydrolysis products without CT explanationFundamental errors in electron counting; misapplies Wade-Mingos rules; states lanthanides show strong colors or broad spectra; fails to identify correct d⁶ species; describes borane reactions incorrectly; attributes color solely to d-d transitions
Mechanism / equation20%10Writes balanced equations for borane reactions with correct products; explains separation mechanism for XeF₆ with chemical equations; describes CT transition mechanism for FeCl₃ color; shows electron flow in M-M bond formationWrites partially correct equations with minor product errors; describes separation qualitatively without chemical basis; mentions CT vaguely without mechanism; shows electron counting without mechanistic insightIncorrect products for borane reactions; no chemical basis for separation; no mention of CT mechanism; missing or wrong equations throughout
Numerical accuracy15%7.5Correctly calculates 1 M-M bond in (a)(i) with clear electron accounting (Fe: 8 + 5 + 2 + 2 = 17 each, needs 1 bond); determines 5 skeletal electron pairs for Bi₅³⁺ leading to n+2 = 7 electrons per Bi, confirming closo structureCorrect final answer with unclear working; or incorrect calculation with partially correct reasoning; confuses electron counting methodsWrong numerical answers for M-M bonds or skeletal pairs; no electron accounting shown; arithmetic errors in simple calculations
Diagram / structure20%10Draws clear structures: Cp₂Fe₂(CO)₄ with bridging/terminal CO and Fe-Fe bond; trigonal bipyramidal Bi₅³⁺ with labeled vertices; ferrocene-type sandwich for (c); depicts CT transition with orbital diagram for (e)Describes structures verbally without diagrams; or draws incomplete/unclear structures; correct geometry mentioned without visualizationNo diagrams where essential; incorrect geometries described; confuses closo/nido/arachno structures; missing orbital diagrams for spectroscopic explanations
Application context20%10Connects to practical applications: noble gas fluorides in nuclear fuel processing (Indian context: BARC research); lanthanide spectroscopy in lasers and phosphors (CSIR-NPL work); organometallics in catalysis; relates FeCl₃ color to analytical detection limits; cites Indian contributions to borane chemistryMentions applications superficially without Indian context; generic references to industrial use; no connection to contemporary researchNo application context provided; fails to relate concepts to real-world uses; missing contemporary or Indian scientific contributions

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