JEE Main 2025 Jan 29 Shift 1, Chemistry Q7: Colour, Magnetism And Stability Of Co-ordination Compounds

JEE Main2025Jan 29, Shift 1Chemistry

The correct increasing order of stability of the complexes based on $Δ_{o}$ value is :

(I) $[Mn (CN)_{6}]^{3 -}$ (II) $[Co (CN)_{6}]^{4 -}$

(III) $[Fe (CN)_{6}]^{4 -}$ (IV) $[Fe (CN)_{6}]^{3 -}$

A
II $<$ III $<$ I $<$ IV
B
IV $<$ III $<$ II $<$ I
C
I $<$ II $<$ IV $<$ III
D
III $<$ II $<$ IV $<$ I

Solution

For low-spin octahedral $CN^{-}$ complexes the crystal-field stabilisation energy (CFSE), expressed as a multiple of $Δ_{o}$ , determines thermodynamic stability. Filling the $t_{2 g}$ set first, each $t_{2 g}$ electron contributes $- 0.4 Δ_{o}$ and each $e_{g}$ electron contributes $+ 0.6 Δ_{o}$ .

(I) $Mn^{3 +}$ is $d^{4}$ (low spin): CFSE $= - 1.6 Δ_{o}$ .

(II) $Co^{2 +}$ is $d^{7}$ (low spin, $t_{2 g}^{6} e_{g}^{1}$ ): CFSE $= - 1.8 Δ_{o}$ .

(III) $Fe^{2 +}$ is $d^{6}$ (low spin, $t_{2 g}^{6}$ ): CFSE $= - 2.4 Δ_{o}$ .

(IV) $Fe^{3 +}$ is $d^{5}$ (low spin, $t_{2 g}^{5}$ ): CFSE $= - 2.0 Δ_{o}$ .

More negative CFSE means greater stability. Increasing stability: I $<$ II $<$ IV $<$ III.

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JEE Main 2025 Jan 29 Shift 1, Chemistry Q7: Colour, Magnetism And Stability Of Co-ordination Compounds

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