Chemistry Q60 – JEE-MAIN 2026

Home Chemistry Coordination CompoundsQuestion #322

Consider the metal complexes

[N i (e n)_{3}]^{2 +}

(A),

[N i C l_{4}]^{2 -}

(B) and

[N i (N H_{3})_{6}]^{2 +}

(C). Choose the CORRECT option by considering the number of unpaired electrons present in (A), (B) and (C) respectively and the order of frequency of absorption.

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

Identify the oxidation state of the central metal ion and its $d$ -electron configuration for all complexes.

🧠 Full Solution Path

Step 1: Determine

d

-electron configuration and geometry✦ Active

In all three complexes, Nickel (Ni) is in the $+ 2$ oxidation state. The electronic configuration of $N i^{2 +}$ is $[A r] 3 d^{8}$ . The geometries are:

(A) [N i (e n)_{3}]^{2 +} : Octahedral

(B) [N i C l_{4}]^{2 -} : Tetrahedral

(C) [N i (N H_{3})_{6}]^{2 +} : Octahedral

Step 2: Calculate the number of unpaired electrons○ Expand

For $d^{8}$ octahedral complexes (A and C), the crystal field configuration is $t_{2 g}^{6} e_{g}^{2}$ . This results in 2 unpaired electrons, regardless of whether the ligand is strong or weak field. For $d^{8}$ tetrahedral complexes (B), the crystal field configuration is $e^{4} t_{2}^{4}$ . Since $Δ_{t}$ is typically small, these are high spin, resulting in 2 unpaired electrons. Therefore, all three complexes (A), (B), and (C) have 2 unpaired electrons.

💡 Teacher's Secret Hint

Remember that $d^{8}$ octahedral complexes are always paramagnetic with 2 unpaired electrons.

Step 3: Determine the order of frequency of absorption○ Expand

The frequency of absorption is directly proportional to the crystal field splitting energy ( $Δ$ ). The order of ligand field strength from the spectrochemical series is $C l^{-} < N H_{3} < e n$ . Also, $Δ_{t}$ is generally much smaller than $Δ_{o}$ . Comparing the complexes:

Δ (B) < Δ (C) < Δ (A)

This is because (B) is tetrahedral with a weak field ligand ( $C l^{-}$ ), (C) is octahedral with a moderate field ligand ( $N H_{3}$ ), and (A) is octahedral with a strong field ligand ( $e n$ ). Thus, the order of frequency of absorption is $(A) > (C) > (B)$ . Combining the results, the correct option is 2, 2, 2 and (A) > (C) > (B).

💡 Teacher's Secret Hint

The spectrochemical series is crucial for comparing ligand field strengths and thus $Δ$ values for similar geometries.

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Chemistry Question 60 – JEE-MAIN 2026

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

🧠 Full Solution Path

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