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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 · 2016 · Paper I

Q5(c). Structure of Fe₂(CO)₉ and the 18-electron rule

  • 10 marks
  • Answered
Question

Write the structure of Fe₂(CO)₉. Show that each Fe atom conform to the 18 electron rule.

Answer in brief

Two Fe(CO)₃ units linked by three bridging CO ligands and an Fe–Fe bond (D₃ₕ). Each Fe: 8 (Fe) + 6 (three terminal CO) + 3 (three μ₂-CO, one electron to each Fe) + 1 (Fe–Fe bond) = 18 electrons.

Key points

  • Each Fe carries three terminal CO ligands; three CO ligands bridge the two Fe atoms (μ₂-CO), so each Fe is surrounded by six CO groups in a distorted octahedron. The two octahedra share a face (the three bridging C atoms).
  • the three bridging CO lie in a plane between the Fe atoms, at 120° to each other about the Fe–Fe axis.
  • The two Fe atoms are close enough for an Fe–Fe bond, needed to give each metal 18 electrons and to explain its diamagnetism (the extent of direct Fe–Fe bonding is debated).
  • terminal CO absorb at 1850–2120 cm⁻¹, bridging CO at 1700–1860 cm⁻¹; Fe₂(CO)₉ shows both kinds. (Uncombined CO: 2143 cm⁻¹.)

The full model answer

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

Structure

  1. Each Fe carries three terminal CO ligands; three CO ligands bridge the two Fe atoms (μ₂-CO), so each Fe is surrounded by six CO groups in a distorted octahedron. The two octahedra share a face (the three bridging C atoms).
  2. The molecule has D₃ₕ symmetry: the three bridging CO lie in a plane between the Fe atoms, at 120° to each other about the Fe–Fe axis.
  3. The two Fe atoms are close enough for an Fe–Fe bond, needed to give each metal 18 electrons and to explain its diamagnetism (the extent of direct Fe–Fe bonding is debated).
  4. IR evidence: terminal CO absorb at 1850–2120 cm⁻¹, bridging CO at 1700–1860 cm⁻¹; Fe₂(CO)₉ shows both kinds. (Uncombined CO: 2143 cm⁻¹.)

18-electron count for each Fe (neutral-atom method)

ContributionElectrons
Fe(0), 3d⁶4s²8
three terminal CO, 2 each6
three μ₂-CO, each shares its 2 electrons between the two Fe: 1 each3
Fe–Fe bond, one electron from the partner Fe1
Total per Fe18
  1. Check for the whole molecule: 2×8+9×2=34 cluster valence electrons =18x-2y with x=2 metals and y=1 M–M bond. Without the Fe–Fe bond each Fe would have 17 electrons and the compound would be paramagnetic; it is diamagnetic.

Application. Fe₂(CO)₉ is made by photolysis of Fe(CO)₅: 2Fe(CO)₅ →Fe₂(CO)₉ + CO. It is a convenient source of the Fe(CO)₄ fragment, e.g. for making (diene)Fe(CO)₃ complexes. Down the group bridging falls off: Ru₂(CO)₉ and Os₂(CO)₉ have only one bridging CO.

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Model answer to Q5(c), 2016 Paper I: solution page 26
Solution page 26 of 46
Read the whole 2016 solution2016 question paper

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