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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 · 2018 · Paper I

Q5(c). Structures of two complexes; IUPAC names of two salts

  • 10 marks
  • Answered
Question

(i) Draw the structure of the following complexes:
(A) bis (η⁶-benzene) chromium (O)
(B) fac-triaquatris (dimethylamine) cobalt (III) tetraoxosulfate (VI)
(ii) Give IUPAC nomenclature of
(A) Na[B(NO₃)₄] (B) [Co(NH₃)₆][CoCl₆]

Answer in brief

(i)(A) Sandwich [Cr(η⁶-C₆H₆)₂], two parallel eclipsed benzene rings, Cr(0), 18 electrons. (B) fac-[Co(H₂O)₃(NHMe₂)₃]₂(SO₄)₃, octahedral, three aqua ligands on one face. (ii)(A) Sodium tetranitratoborate(III); (B) hexaamminecobalt(III) hexachloridocobaltate(III).

Key points

  • Cr lies between two parallel benzene rings, each bonded through all six carbon atoms (η⁶, hapticity 6).
  • The rings are eclipsed (D₆ₕ), all C–C bonds of a ring equal. Electron count: Cr(0) d⁶ (6) + two benzene (6 + 6) = 18 electrons; diamagnetic, like ferrocene.
  • The cation [Co(H₂O)₃(NH(CH₃)₂)₃]³⁺ is octahedral Co(III), all ligands neutral, so its charge is +3. Tetraoxosulfate(VI) is SO₄²⁻; charge balance needs two cations to three anions: [Co(H₂O)₃(NHMe₂)₃]₂(SO₄)₃
  • the three H₂O occupy the three corners of one triangular face, all mutually cis; the three NHMe₂ occupy the opposite face. (In the mer isomer, three like ligands lie in one plane through Co, two of them trans.)

The full model answer

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

Reading: “chromium (O)” is chromium(0).

(i)(A) Bis(η⁶-benzene)chromium(0)

  1. Sandwich compound: Cr lies between two parallel benzene rings, each bonded through all six carbon atoms (η⁶, hapticity 6).
  2. The rings are eclipsed (D₆ₕ), all C–C bonds of a ring equal. Electron count: Cr(0) d⁶ (6) + two benzene (6 + 6) = 18 electrons; diamagnetic, like ferrocene.

(i)(B) fac-Triaquatris(dimethylamine)cobalt(III) tetraoxosulfate(VI)

  1. The cation [Co(H₂O)₃(NH(CH₃)₂)₃]³⁺ is octahedral Co(III), all ligands neutral, so its charge is +3. Tetraoxosulfate(VI) is SO₄²⁻; charge balance needs two cations to three anions: [Co(H₂O)₃(NHMe₂)₃]₂(SO₄)₃
  2. fac (facial): the three H₂O occupy the three corners of one triangular face, all mutually cis; the three NHMe₂ occupy the opposite face. (In the mer isomer, three like ligands lie in one plane through Co, two of them trans.)

(ii) IUPAC names

  1. (A) Na[B(NO₃)₄]: the complex anion has four nitrate ligands (nitrato, bound through O) on B; charge -1, so B is +3: 2pt] sodium tetranitratoborate(III) (2005 form: sodium tetrakis(nitrato-κ O)borate(1−)). The anion is tetrahedral.
  2. (B) [Co(NH₃)₆][CoCl₆]: both ions are complex. Cation [Co(NH₃)₆]³⁺ (Co +3); anion [CoCl₆]³⁻ (Co +3), named with -ate: 2pt] hexaamminecobalt(III) hexachloridocobaltate(III) (older: hexachlorocobaltate(III)). Ammine is spelt with double m for NH₃.
  3. Salts with complex cation and complex anion show coordination isomerism: the ligands can exchange between the two metal centres.

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Model answer to Q5(c), 2018 Paper I: solution page 32
Solution page 32 of 55
Read the whole 2018 solution2018 question paper

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