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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2019 · Paper I

Q8(c). Structure of ferrocene and its MO description

  • 10 marks
  • Answered
Question

Depict the structure of ferrocene and describe the bonding of it by Molecular orbital theory including orbital diagrams.

Answer in brief

Ferrocene, [Fe(η⁵-C₅H₅)₂], is a sandwich: Fe(II) between two parallel C₅H₅⁻ rings. The ring π combinations of e_(1g) symmetry overlap Fe d_(xz),d_(yz) to give the main Fe–ring bond; the 18 valence electrons fill nine bonding and nearly non-bonding MOs, the highest being e_(2g) and a_(1g) (d_(z²)).

Key points

  • two planar, parallel C₅H₅ rings with Fe midway on the fivefold axis; all ten C atoms equally bonded (η⁵). In the gas phase the rings are eclipsed (D₅ₕ); the staggered form (D_(5d)) lies only slightly higher and the rings…
  • each C₅H₅⁻ has five π MOs: a (no node, filled), e₁ (one node, filled), e₂ (two nodes, empty). For two rings, in-phase and out-of-phase sums give a_(1g)+a₂ᵤ, e_(1g)+e₁ᵤ (12 π electrons) and empty e_(2g)+e₂ᵤ.
  • d_(z²) and 4s (a_(1g)), d_(xz),d_(yz) (e_(1g)), d_(x²-y²),d_(xy) (e_(2g)), 4p_z (a₂ᵤ), 4pₓ,4p_y (e₁ᵤ). Only orbitals of the same symmetry combine.
  • the filled ligand e_(1g) set overlaps Fe d_(xz),d_(yz) strongly (π-type): the e_(1g) bonding MOs carry most of the Fe–ring bonding. The ligand a_(1g), a₂ᵤ and e₁ᵤ combinations bond more weakly with Fe 4s and 4p.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Structure: two planar, parallel C₅H₅ rings with Fe midway on the fivefold axis; all ten C atoms equally bonded (η⁵). In the gas phase the rings are eclipsed (D₅ₕ); the staggered form (D_(5d)) lies only slightly higher and the rings rotate rapidly at room temperature. MO labels below use the centrosymmetric D_(5d) form.
  2. Ligand orbitals: each C₅H₅⁻ has five π MOs: a (no node, filled), e₁ (one node, filled), e₂ (two nodes, empty). For two rings, in-phase and out-of-phase sums give a_(1g)+a₂ᵤ, e_(1g)+e₁ᵤ (12 π electrons) and empty e_(2g)+e₂ᵤ.
  3. Metal orbitals: d_(z²) and 4s (a_(1g)), d_(xz),d_(yz) (e_(1g)), d_(x²-y²),d_(xy) (e_(2g)), 4p_z (a₂ᵤ), 4pₓ,4p_y (e₁ᵤ). Only orbitals of the same symmetry combine.
  4. Main bond: the filled ligand e_(1g) set overlaps Fe d_(xz),d_(yz) strongly (π-type): the e_(1g) bonding MOs carry most of the Fe–ring bonding. The ligand a_(1g), a₂ᵤ and e₁ᵤ combinations bond more weakly with Fe 4s and 4p.
  5. Metal-centred levels: e_(2g) (d_(x²-y²),d_(xy), slight δ back-donation to the empty ring e_(2g)) and a_(1g) (d_(z²), pointing at the ring centres where the π density is low, so nearly non-bonding) hold the six metal electrons; a_(1g) is the HOMO. The LUMO is the antibonding e*_(1g).
  6. Filling: 18 electrons fill a_(1g), a₂ᵤ, e₁ᵤ, e_(1g), e_(2g), a_(1g) (9 MOs); all are paired: diamagnetic, stable, 18-electron. Removing one electron from the HOMO gives the blue, paramagnetic ferrocenium ion [Fe(C₅H₅)₂]⁺.

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Model answer to Q8(c), 2019 Paper I: solution page 38
Solution page 38 of 40
Read the whole 2019 solution2019 question paper

All parts of Question 8