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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

Q6(c)(i). Geometry of [Ni(H₂O)₆]²⁺ by valence bond theory

  • 5 marks
  • Answered
Question

Determine the geometry of [Ni(H₂O)₆]²⁺ (paramagnetic complex with two unpaired electrons) using valence bond theory.

Answer in brief

Ni(II) is 3d⁸ with two unpaired electrons kept; the six H₂O lone pairs enter 4s, 4p³ and 4d²: sp³d² (outer-orbital) hybridisation, octahedral, μₛ=2.83 B.M.

Key points

  • [Ar]3d⁸4s². Ni²⁺: [Ar]3d⁸, with two unpaired electrons in the e_g-type orbitals.
  • Six ligands need six empty hybrid orbitals. Even if the 3d electrons were paired, only one 3d orbital would be empty, not the two needed for d²sp³. So the inner 3d orbitals cannot be used.
  • The six water lone pairs go into 4s, three 4p and two 4d orbitals: sp³d² hybrids pointing to the corners of an octahedron. The complex is an outer-orbital (high-spin type) complex.
  • The two 3d electrons stay unpaired, as the question states: μₛ=√(n(n+2))=√8 μₛ=2.83 B.M., octahedral

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. Ni (Z=28): [Ar]3d⁸4s². Ni²⁺: [Ar]3d⁸, with two unpaired electrons in the e_g-type orbitals.
  2. Six ligands need six empty hybrid orbitals. Even if the 3d electrons were paired, only one 3d orbital would be empty, not the two needed for d²sp³. So the inner 3d orbitals cannot be used.
  3. The six water lone pairs go into 4s, three 4p and two 4d orbitals: sp³d² hybrids pointing to the corners of an octahedron. The complex is an outer-orbital (high-spin type) complex.
  4. The two 3d electrons stay unpaired, as the question states: μₛ=√(n(n+2))=√8 μₛ=2.83 B.M., octahedral
  5. Water is a weak-field ligand; a strong field instead gives square planar dsp², diamagnetic [Ni(CN)₄]²⁻.

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Model answer to Q6(c)(i), 2018 Paper I: solution page 38
Solution page 38 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 6