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Solved 2025: FYUG Even Semester Exam, 2025 CHEMISTRY (CHMDSC-252) Organometallic and Analytical Chemistry

Subject: Chemistry

Course No: CHMDSC-252

Semester: 4th Semester (FYUG)

Exam Name: Even Semester Exam, 2025

Full Marks: 70 | Pass Marks: 28

Time: 3 Hours

UNIT-I

1. (a) What is haptacity? Mention haptacity in the given compounds. 4

Definition: Hapticity (denoted by η) refers to the number of contiguous atoms of a ligand that are directly coordinated to a central metal atom

.

Analysis:

  • In the first structure (Allyl group coordinated via one double bond and a radical), the hapticity is η³.
  • In the second structure (Benzene ring), the hapticity is η⁶.

    • 1. (b) What is 18-electron rule? Give examples. 4

    The 18-electron rule states that thermodynamically stable transition metal organometallic compounds are formed when the sum of the metal's valence electrons plus the electrons donated by the ligands equals 18, achieving the noble gas configuration.

    Examples:

  • Fe(CO)₅: Fe (8) + 5(2 from CO) = 18 electrons.
  • Cr(CO)₆: Cr (6) + 6(2 from CO) = 18 electrons.

    • 1. (c) Draw the structures of eclipsed and staggered ferrocene. 4

    Ferrocene Fe(C₅H₅)₂ exists in two main conformations

    :
  • Eclipsed: The cyclopentadienyl rings are directly aligned one above the other (D₅h symmetry).
  • Staggered: One ring is rotated 36° relative to the other (D₅d symmetry).

    • 2. (a) (i) Count electrons as per EAN rule. 4

    Molecules:

  • [V(CO)₆]⁻: V (23) + 1 (charge) + 6(2) = 36 (Kr configuration).
  • [Rh(PPh₃)₃Cl]: Rh (45) + 3(2) + 1(1) = 52 (Does not follow EAN/18e rule, usually 16e).
    • 2. (a) (ii) 16-electron species: Wilkinson's Catalyst [RhCl(PPh₃)₃].

    UNIT-II

    4. (a) Mention one method of preparation of Grignard reagent. 2

    Grignard reagents are prepared by the reaction of an alkyl or aryl halide with magnesium metal in the presence of dry ether

    .

    R-X + Mg → R-Mg-X (in dry ether)

    4. (b) Why is dry ether used in the preparation of Grignard reagent? 2

    Dry ether acts as a solvent and a Lewis base that stabilizes the Grignard reagent through coordination with the magnesium atom

    . "Dry" (anhydrous) conditions are essential because Grignard reagents are highly reactive and will react with water to form alkanes.

    UNIT-III

    7. (b) What are inert and labile complexes? 2

  • Labile Complexes: Those in which ligand substitution reactions occur rapidly (usually within 1 minute at 25°C).
  • Inert Complexes: Those in which ligand substitution reactions are very slow.

    • 7. (c) What is trans-effect? 2

    The trans-effect is the labilization of a ligand trans to a certain other ligand (the trans-director) in square planar complexes, which speeds up the substitution of that trans-ligand.

    UNIT-IV

    10. (a) Why is Wilkinson catalyst called homogeneous catalyst? 2

    It is called a homogeneous catalyst because the catalyst and the reactants (alkene and H₂) are in the same phase (liquid/solution) during the reaction

    .

    12. (c) Mention hybridization and geometry of Wilkinson catalyst. 3

    Hybridization: dsp²

    .

    Geometry: Square Planar

    .

    UNIT-V

    13. (b) Discuss solubility product and its relation with precipitation. 2

    The Solubility Product (Ksp) is the equilibrium constant for a solid substance dissolving in an aqueous solution

    . Precipitation occurs when the Ionic Product (Q) exceeds the Ksp (Q > Ksp).

    Exam Focus Enhancements

    Exam Tips

  • Always check the oxidation state of the metal before counting electrons for the 18-electron rule.
  • Memorize the trans-directing series for square planar substitution questions.

    • Common Mistakes

  • Forgetting to specify "dry" ether for Grignard reactions; moisture destroys the reagent.
  • Confusing EAN (total electrons) with the 18-electron rule (valence electrons).

    • Important Formulas List

  • EAN = Z - (Oxidation State) + 2 * (Number of Ligands)
  • 18e Sum = Metal Valence e⁻ + Ligand Donor e⁻
  • Ksp = [A⁺]ⁿ[B⁻]ᵐ
    • xxx

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