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Coordination Compounds question

2002 · Shift 0 · Q49
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Coordination Compounds question

2002 · Shift 0 · Q49

JEE MainChemistryCoordination CompoundsMCQ+4 / −1
The most stable ion is :
  1. A
    [Fe(OH)3]3−[Fe(OH)_3]^{3-}[Fe(OH)3​]3−
  2. B
    [Fe(Cl)6]3−[Fe(Cl)_6]^{3-}[Fe(Cl)6​]3−
  3. C
    [Fe(CN)6]3−[Fe(CN)_6]^{3-}[Fe(CN)6​]3−
  4. D
    [Fe(H2O)6]3+[Fe(H_2O)_6]^{3+}[Fe(H2​O)6​]3+
View written solutionFree

Correct answer: C

  1. Identify the metal ion and its oxidation state

All the given complexes contain iron in the oxidation state +3+3+3:

  • [Fe(OH)3]3−[Fe(OH)_3]^{3-}[Fe(OH)3​]3−
  • [Fe(Cl)6]3−[Fe(Cl)_6]^{3-}[Fe(Cl)6​]3−
  • [Fe(CN)6]3−[Fe(CN)_6]^{3-}[Fe(CN)6​]3−
  • [Fe(H2O)6]3+[Fe(H_2O)_6]^{3+}[Fe(H2​O)6​]3+

So we are comparing the relative stability of different Fe3+Fe^{3+}Fe3+ complexes with different ligands.

  1. Use the concept of complex stability

The stability of coordination complexes depends strongly on the nature of the ligand.

According to the spectrochemical series:

CN−>H2O>Cl−CN^- > H_2O > Cl^-CN−>H2​O>Cl−

Also, OH−OH^-OH− can bind strongly, but for Fe3+Fe^{3+}Fe3+ the cyanide complex is exceptionally stable because of:

  • strong ligand field strength,
  • good overlap with metal orbitals,
  • significant stabilization of the complex.
  1. Compare the ligands one by one
  • Cl−Cl^-Cl− is a weak-field ligand, so [FeCl6]3−[FeCl_6]^{3-}[FeCl6​]3− is not the most stable.
  • H2OH_2OH2​O is stronger than Cl−Cl^-Cl− but still not as stabilizing as CN−CN^-CN−.
  • OH−OH^-OH− can coordinate, but iron(III) hydroxide systems tend to undergo hydrolysis/precipitation behavior rather than forming the most stable complex ion among these choices.
  • CN−CN^-CN− is a very strong-field ligand and forms highly stable complexes with Fe3+Fe^{3+}Fe3+.

Thus, [Fe(CN)6]3−[Fe(CN)_6]^{3-}[Fe(CN)6​]3− is the most stable ion.

  1. Check the correct option

[Fe(CN)6]3−\boxed{[Fe(CN)_6]^{3-}}[Fe(CN)6​]3−​

So the correct option is C.

  1. Comparison with stored answer

Stored correct answer: C

Our derived answer: C

Hence, they agree.

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