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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

Q8(d). Stereoisomers of [PtCl₂Br₂(NH₃)₂]

  • 10 marks
  • Answered
Question

Draw all possible stereoisomers of [PtCl₂Br₂(NH₃)₂].

Answer in brief

Octahedral Pt(IV), type Ma₂b₂c₂: five geometrical isomers, one of which (all cis) is chiral and exists as a pair of enantiomers: six stereoisomers in all.

Key points

  • x+2(-1)+2(-1)+0=0, so Pt(IV), 5d⁶, low spin, octahedral. Three pairs of like monodentate ligands: Ma₂b₂c₂.
  • Classify by which pairs are trans. Each pair can be trans (180°) or cis (90°), but if two pairs are trans the third must be trans too.
  • The all-cis isomer has no plane or centre of symmetry (point group C₁); it and its mirror image are non-superimposable: a pair of enantiomers, optically active.
  • 5 geometrical isomers; 6 stereoisomers (6!/(2! 2! 2!)=90 ways of placing the ligands on the six corners reduce, under the rotations of the octahedron, to these six).

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. Oxidation state: x+2(-1)+2(-1)+0=0, so Pt(IV), 5d⁶, low spin, octahedral. Three pairs of like monodentate ligands: Ma₂b₂c₂.
  2. Classify by which pairs are trans. Each pair can be trans (180°) or cis (90°), but if two pairs are trans the third must be trans too.
IsomerArrangementTrans pairsMirror plane?Chiral?
1Cl trans Cl, Br trans Br, NH₃ trans NH₃ (all trans)3yesno
2Cl trans Cl; Br and NH₃ pairs cis1yesno
3Br trans Br; Cl and NH₃ pairs cis1yesno
4NH₃ trans NH₃; Cl and Br pairs cis1yesno
5, 5'all three pairs cis (each Cl trans to Br or NH₃)0no
  1. The all-cis isomer has no plane or centre of symmetry (point group C₁); it and its mirror image are non-superimposable: a pair of enantiomers, optically active.
  2. 5 geometrical isomers; 6 stereoisomers (6!/(2! 2! 2!)=90 ways of placing the ligands on the six corners reduce, under the rotations of the octahedron, to these six).
  3. The same count holds for every Ma₂b₂c₂ octahedral complex, e.g. [Co(CN)₂(H₂O)₂(NH₃)₂]⁺.

Application. the chiral all-cis form can be resolved with an optically active counter-ion; Werner's resolution of such octahedral complexes proved the octahedral geometry of six-coordinate metal ions.

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Model answer to Q8(d), 2018 Paper I: solution page 53
Solution page 53 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 8