The correct order of spin-only magnetic moments for the following… — Solution | JEE Chemistry

MCQMediumJEE 2023Crystal Field Theory

The correct order of spin-only magnetic moments for the following complex ions is:

Answer

Correct answer:D

Standard Method

Given: The complexes are $[CoF_6]^{3-}$ , $[MnBr_4]^{2-}$ , $[Fe(CN)_6]^{3-}$ and $[Mn(CN)_6]^{3-}$ .

Find: The correct increasing order of their spin-only magnetic moments.

The spin-only magnetic moment depends on the number of unpaired electrons.

For $[CoF_6]^{3-}$ :

Co^{3+} = d^6

Ligand $F^-$ is weak field, so pairing does not occur. Therefore unpaired electrons $= 4$ .

For $[MnBr_4]^{2-}$ :

Mn^{2+} = d^5

Ligand $Br^-$ is weak field, so no pairing occurs. Therefore unpaired electrons $= 5$ .

For $[Fe(CN)_6]^{3-}$ :

Fe^{3+} = d^5

Ligand $CN^-$ is strong field, so pairing occurs. Therefore unpaired electrons $= 1$ .

For $[Mn(CN)_6]^{3-}$ :

Mn^{3+} = d^4

Ligand $CN^-$ is strong field, so pairing occurs. Therefore unpaired electrons $= 2$ .

Hence, increasing order of magnetic moment is:

[Fe(CN)_6]^{3-} < [Mn(CN)_6]^{3-} < [CoF_6]^{3-} < [MnBr_4]^{2-}

Therefore, the correct option is D.

Treating $CN^-$ as a weak field ligand is incorrect because it is a strong field ligand and causes pairing. Use ligand field strength correctly before counting unpaired electrons.
Ignoring the oxidation state while finding the $d$ -electron count is wrong. First determine the metal oxidation state, then write the correct configuration such as $Fe^{3+} = d^5$ or $Mn^{3+} = d^4$ .
Comparing magnetic moments without counting unpaired electrons is incorrect. Spin-only magnetic moment increases with the number of unpaired electrons, so determine the unpaired electrons for each complex first.

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