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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper I covers inorganic chemistry.

UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper I

Q1(g). First-row metal in EAN-obeying organometallics

  • 5 marks
  • Topic: Organometallic chemistry
  • Answered
Question

Identify the first row transition metal in the following organometallic compounds obeying EAN rule:
(i) [M(CO)₃(PPh₃)]⁻ (ii) [M(η¹-C₅H₅)(η⁵-C₅H₅)(CO)₂] (iii) [M(η⁵-C₅H₅)(CO)₃]₂

Answer in brief

(i) Co (ii) Fe (iii) Cr

Key points

  • (i) is the tetrahedral [Co(CO)₃(PPh₃)]⁻, analogue of [Co(CO)₄]⁻. (ii) is the fluxional Fe(η¹-Cp)(η⁵-Cp)(CO)₂. (iii) is [CpCr(CO)₃]₂, Cr–Cr single bond (3.28 Å).

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

EAN rule: metal valence electrons + ligand electrons (neutral counting) =18 (36 for the Kr core).

CompoundElectron countx (metal)M
[M(CO)₃(PPh₃)]⁻x+3(2)+2+1 (charge) =189Co
[M(η¹-Cp)(η⁵-Cp)(CO)₂]x+1+5+2(2)=188Fe
[M(η⁵-Cp)(CO)₃]₂per metal: x+5+3(2)+1 (M–M bond) =186Cr
  • (i) is the tetrahedral [Co(CO)₃(PPh₃)]⁻, analogue of [Co(CO)₄]⁻. (ii) is the fluxional Fe(η¹-Cp)(η⁵-Cp)(CO)₂. (iii) is [CpCr(CO)₃]₂, Cr–Cr single bond (3.28 Å).

the 18-electron rule fails for early metals (bulky ligands) and for square-planar d⁸ ions (16 electrons), but it holds for carbonyls.

Model answer to Q1(g), 2026 Mains Paper I: solution page 6
Solution page 6 of 26
Read the whole 2026 solution2026 question paper

Covers18-electron ruleFirst-row metal in EAN-obeying organometallics

All parts of Question 1