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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 · 2017 · Paper I

Q8(a)(i). EAN rule applied to [Fe(CN)₆]⁴- and [Fe(CN)₆]³-

  • 10 marks
  • Answered
Question

What is meant by EAN rule applied to the complexes? Apply the concept of EAN rule on the following two complexes [Fe(CN)₆]⁴- and [Fe(CN)₆]³- and draw your conclusion about the validity of the rule.

Answer in brief

EAN = electrons on the metal ion + 2 × (donor pairs). [Fe(CN)₆]⁴-: 24+12=36 (Kr), obeys. [Fe(CN)₆]³-: 23+12=35, does not, yet it is a stable, inert complex. The rule is not a necessary condition for stability.

Key points

  • EAN rule (Sidgwick). The effective atomic number of the metal in a complex is the total number of electrons around it: the electrons of the metal ion plus two from each ligand donor atom. The rule says a stable complex has an…
  • [Fe(CN)₆]⁴-: Fe (Z=26) is Fe(II): 26-2=24 electrons; six CN⁻ donate 12. EAN=24+12=36 = Kr. The rule is obeyed; the complex is very stable and inert (18 valence electrons, low-spin t_(2g)⁶).
  • [Fe(CN)₆]³-: Fe(III): 26-3=23; ligands 12. EAN=35 (17 valence electrons). The rule is not obeyed, yet K₃[Fe(CN)₆] is a stable, well-known complex, low spin t_(2g)⁵, one unpaired electron.
  • Conclusion on validity. Obeying the rule is not necessary for stability: [Fe(CN)₆]³- (35), [Co(NH₃)₆]²+ (37), [Ni(NH₃)₆]²+ (38) and [Cr(NH₃)₆]³+ (33) are all stable. The rule is only a guide; it holds well for metal carbonyls…

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. EAN rule (Sidgwick). The effective atomic number of the metal in a complex is the total number of electrons around it: the electrons of the metal ion plus two from each ligand donor atom. The rule says a stable complex has an EAN equal to the atomic number of the next noble gas. In valence-shell terms this is the 18-electron rule.
  2. [Fe(CN)₆]⁴-: Fe (Z=26) is Fe(II): 26-2=24 electrons; six CN⁻ donate 12. EAN=24+12=36 = Kr. The rule is obeyed; the complex is very stable and inert (18 valence electrons, low-spin t_(2g)⁶).
  3. [Fe(CN)₆]³-: Fe(III): 26-3=23; ligands 12. EAN=35 (17 valence electrons). The rule is not obeyed, yet K₃[Fe(CN)₆] is a stable, well-known complex, low spin t_(2g)⁵, one unpaired electron.
  4. Conclusion on validity. Obeying the rule is not necessary for stability: [Fe(CN)₆]³- (35), [Co(NH₃)₆]²+ (37), [Ni(NH₃)₆]²+ (38) and [Cr(NH₃)₆]³+ (33) are all stable. The rule is only a guide; it holds well for metal carbonyls and organometallic compounds with strong π-acceptor ligands (Ni(CO)₄, Fe(CO)₅, Cr(CO)₆: EAN 36), where the 18-electron count fills all bonding and non-bonding MOs.
  5. Why it fails here: in octahedral complexes the t_(2g) set is only weakly bonding (non-bonding for σ donors), so removing one electron from it (Fe(II) → Fe(III)) costs little stability. Stability depends on the metal–ligand bond strength, CFSE and charge, not on a noble-gas count.

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Model answer to Q8(a)(i), 2017 Paper I: solution page 33
Solution page 33 of 38
Read the whole 2017 solution2017 question paper

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