Chemistry Q62 – JEE-MAIN 2025

Home Chemistry Coordination CompoundsQuestion #351

The number of unpaired electrons responsible for the paramagnetic nature of the following complex species are respectively :

[F e (C N)_{6}]^{3 -}

[F e F_{6}]^{3 -}

[C o F_{6}]^{3 -}

[M n (C N)_{6}]^{3 -}

Clue 1 of 3: Determine Oxidation State and d-electron Count

First, find the oxidation state of the central metal atom in each complex and then determine its d-electron configuration.

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Ninja Strategy•Sequential Ligand Field Analysis

Determine the oxidation state and d-electron count for each metal, then apply Crystal Field Theory considering strong or weak field ligands sequentially to eliminate options.

🧠 Full Solution Path

Step 1: Determine oxidation state and d-electron count for each metal ion✦ Active

For each complex, calculate the oxidation state of the central metal atom and determine its d-electron configuration:

- For $[F e (C N)_{6}]^{3 -}$ : $F e^{3 +}$ ( $3 d^{5}$ )

- For $[F e F_{6}]^{3 -}$ : $F e^{3 +}$ ( $3 d^{5}$ )

- For $[C o F_{6}]^{3 -}$ : $C o^{3 +}$ ( $3 d^{6}$ )

- For $[M n (C N)_{6}]^{3 -}$ : $M n^{3 +}$ ( $3 d^{4}$ )

Step 2: Apply Crystal Field Theory based on ligand strength to find unpaired electrons○ Expand

Consider the ligand field strength (strong field or weak field) and distribute the d-electrons into $t_{2 g}$ and $e_{g}$ orbitals for octahedral complexes:

- ** $[F e (C N)_{6}]^{3 -}$ :** $F e^{3 +}$ ( $3 d^{5}$ ). $C N^{-}$ is a strong field ligand. Electrons pair up. Configuration: $t_{2 g}^{5} e_{g}^{0}$ . Number of unpaired electrons = 1.

- ** $[F e F_{6}]^{3 -}$ :** $F e^{3 +}$ ( $3 d^{5}$ ). $F^{-}$ is a weak field ligand. Electrons do not pair up. Configuration: $t_{2 g}^{3} e_{g}^{2}$ . Number of unpaired electrons = 5.

- ** $[C o F_{6}]^{3 -}$ :** $C o^{3 +}$ ( $3 d^{6}$ ). $F^{-}$ is a weak field ligand. Electrons do not pair up. Configuration: $t_{2 g}^{4} e_{g}^{2}$ . Number of unpaired electrons = 4.

- ** $[M n (C N)_{6}]^{3 -}$ :** $M n^{3 +}$ ( $3 d^{4}$ ). $C N^{-}$ is a strong field ligand. Electrons pair up. Configuration: $t_{2 g}^{4} e_{g}^{0}$ . Number of unpaired electrons = 2.

💡 Teacher's Secret Hint

Remember that strong field ligands (like $C N^{-}$ ) cause pairing, leading to low spin complexes, while weak field ligands (like $F^{-}$ ) do not, leading to high spin complexes.

Step 3: Compile the results and select the correct option○ Expand

The number of unpaired electrons for the complexes $[F e (C N)_{6}]^{3 -}$ , $[F e F_{6}]^{3 -}$ , $[C o F_{6}]^{3 -}$ , and $[M n (C N)_{6}]^{3 -}$ are $1, 5, 4, 2$ respectively. This sequence matches option 4.

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Chemistry Question 62 – JEE-MAIN 2025

Clue 1 of 3: Determine Oxidation State and d-electron Count

🧠 Full Solution Path