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JEE Chemistry 2023 Question with Solution

The correct order of the number of unpaired electrons in the given complexes is

A. [Fe(CN)6]3−[Fe(CN)_6]^{3-}

B. [FeF6]3−[FeF_6]^{3-}

C. [CoF6]3−[CoF_6]^{3-}

D. [Cr(oxalate)3]3−[Cr(oxalate)_3]^{3-}

E. [Ni(CO)4][Ni(CO)_4]

Choose the correct answer from the options given below:

  • A

    E << A << D << C << B

  • B

    A << E << C << B << D

  • C

    A << E << D << C << B

  • D

    E << A << B << D << C

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: The complexes are A = [Fe(CN)6]3−[Fe(CN)_6]^{3-}, B = [FeF6]3−[FeF_6]^{3-}, C = [CoF6]3−[CoF_6]^{3-}, D = [Cr(oxalate)3]3−[Cr(oxalate)_3]^{3-} and E = [Ni(CO)4][Ni(CO)_4].

Find: The correct increasing order of the number of unpaired electrons.

From the solution, the relevant metal electronic configurations are identified as:

Fe3+=3d5\text{Fe}^{3+} = 3d^5

for A and B,

Co3+\text{Co}^{3+}

for C,

Cr3+\text{Cr}^{3+}

for D and

Ni\text{Ni}

for E.

Now use ligand field strength.

  • CN−^- and CO are strong field ligands, so pairing is favored.
  • F−^- is a weak field ligand, so high-spin configurations are favored.
  • Oxalate gives the stated intermediate result from the extracted solution.

Therefore, after considering weak field and strong field effects, the order of unpaired electrons given in the solution is

E<A<D<C<BE < A < D < C < B

This sequence matches option C in the given list. Therefore, the correct option is C.

Common mistakes

  • Treating all ligands as having the same field strength is incorrect because the number of unpaired electrons depends strongly on whether the ligand is strong field or weak field. First identify the ligand type, then decide high spin or low spin.

  • Using the neutral metal electronic configuration instead of the oxidation state present in the complex is wrong. Determine the oxidation state of the metal ion in each complex before writing its dd-electron count.

  • Assuming [Ni(CO)4][Ni(CO)_4] has unpaired electrons is incorrect because CO is a strong field ligand and this complex is diamagnetic. Check the final electron arrangement rather than only the elemental configuration of nickel.

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