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MCQEasyJEE 2023Crystal Field Theory

JEE Chemistry 2023 Question with Solution

Which of the following complex will show largest splitting of d-orbitals?

  • A

    [Fe(C2O4)3]3−[Fe(C_2O_4)_3]^{3-}

  • B

    [Fe6(CN)6][Fe_6(CN)_6]

  • C

    [Fe(CN)6]3−[Fe(CN)_6]^{3-}

  • D

    [Fe(NH3)6]3+[Fe(NH_3)_6]^{3+}

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: We must identify which complex shows the largest splitting of d-orbitals.

Find: The complex with maximum crystal field splitting.

The splitting of d-orbitals in a complex depends on the strength of the ligand according to the spectrochemical series. Strong field ligands produce larger splitting, while weak field ligands produce smaller splitting.

From the given explanation:

  1. [Fe(C2O4)3]3−[Fe(C_2O_4)_3]^{3-} contains oxalate, which is a moderately strong field ligand.
  2. The solution states option B, but its working discusses [FeF6]3−[FeF_6]^{3-} as a weak field complex, whereas the listed option B is [Fe6(CN)6][Fe_6(CN)_6]. This indicates a source discrepancy.
  3. [Fe(CN)6]3−[Fe(CN)_6]^{3-} contains cyanide, and CN−CN^- is a very strong field ligand.
  4. [Fe(NH3)6]3+[Fe(NH_3)_6]^{3+} contains ammonia, which is a moderate field ligand.

Therefore, the complex with the largest splitting is [Fe(CN)6]3−[Fe(CN)_6]^{3-}.

The correct option is C.

Ligand Strength Comparison

Given: Crystal field splitting depends mainly on ligand field strength.

Find: Which ligand among oxalate, cyanide, and ammonia produces the greatest splitting.

Using the spectrochemical series, cyanide is stronger than ammonia and oxalate, while fluoride is weak field.

So the relative effect is:

CN−>NH3>C2O42−>F−CN^- > NH_3 > C_2O_4^{2-} > F^-

Hence the complex containing CN−CN^- will show the largest splitting.

Thus, among the listed options, [Fe(CN)6]3−[Fe(CN)_6]^{3-} is the most defensible answer, even though the source solution incorrectly labels the option letter as B while naming the cyanide complex as the correct complex.

Therefore, the correct option is C.

Common mistakes

  • Confusing the option letter with the complex named in the solution. The solution text says option B, but the explained complex is [Fe(CN)6]3−[Fe(CN)_6]^{3-}, which is actually option C in the listed choices. Always trust the chemical reasoning and match it with the actual option list.

  • Assuming ammonia is stronger than cyanide. This is wrong because CN−CN^- is a much stronger field ligand than NH3NH_3 in the spectrochemical series. Compare ligand strengths before selecting the complex.

  • Judging splitting only from the metal ion and ignoring the ligand. Here the metal is iron in all meaningful choices, so the dominant comparison comes from ligand field strength. Focus on the ligand when the central metal is essentially the same.

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