JEE Main 2025 Jan 22 Shift 1, Chemistry Q16: Colour, Magnetism And Stability Of Co-ordination Compounds

JEE Main2025Jan 22, Shift 1Chemistry

In which of the following complexes the CFSE, $Δ_{0}$ will be equal to zero?

A
[Fe(NH $_{3}$ ) $_{6}$ ]Br $_{2}$
B
[Fe(en) $_{3}$ ]Cl $_{3}$
C
K $_{4}$ [Fe(CN) $_{6}$ ]
D
K $_{3}$ [Fe(SCN) $_{6}$ ]

Solution

In K $_{3}$ [Fe(SCN) $_{6}$ ], Fe is in the +3 state with $d^{5}$ configuration. SCN $^{-}$ (N-bonded thiocyanate) acts as a weak-field ligand, giving a high-spin $d^{5}$ arrangement: $t_{2 g}^{3} e_{g}^{2}$ .

CFSE $= [- 0.4 \times 3 + 0.6 \times 2] Δ_{0} = [- 1.2 + 1.2] Δ_{0} = 0$ .

The other complexes ([Fe(NH $_{3}$ ) $_{6}$ ] $^{2 +}$ is $d^{6}$ ; [Fe(en) $_{3}$ ] $^{3 +}$ is low-spin $d^{5}$ with en a moderate field; [Fe(CN) $_{6}$ ] $^{4 -}$ is low-spin $d^{6}$ ) all have non-zero CFSE.

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JEE Main 2025 Jan 22 Shift 1, Chemistry Q16: Colour, Magnetism And Stability Of Co-ordination Compounds

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