The correct increasing order of stability of the complexes based on \(\Delta_{\mathrm{o}}\) value is : (I) \(\left[\mathrm{Mn}(\mathrm{CN})_{6}\right]^{3-}\) (II) \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{4-}\) (III) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\) (IV) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\)

JEE Main · 2025 · 29 Jan 2025 · Shift 1 · Coordination Compounds

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 -}

Answer: (C)

All four complexes are octahedral with the strong-field ligand

CN^{-}

, so each metal centre is low-spin, and complex stability tracks the magnitude of the crystal field stabilisation energy (CFSE).

(I)

[Mn (CN)_{6}]^{3 -}

Mn^{3 +}

d^{4}

, low-spin

t_{2 g}^{4} e_{g}^{0}

, CFSE

= - 1.6 Δ_{o}

.
(II)

[Co (CN)_{6}]^{4 -}

Co^{2 +}

d^{7}

, low-spin

t_{2 g}^{6} e_{g}^{1}

, CFSE

= - 1.8 Δ_{o}

.
(IV)

[Fe (CN)_{6}]^{3 -}

Fe^{3 +}

d^{5}

, low-spin

t_{2 g}^{5} e_{g}^{0}

, CFSE

= - 2.0 Δ_{o}

.
(III)

[Fe (CN)_{6}]^{4 -}

Fe^{2 +}

d^{6}

, low-spin

t_{2 g}^{6} e_{g}^{0}

, CFSE

= - 2.4 Δ_{o}

, the largest stabilisation of the set.

Increasing order of stability follows increasing magnitude of CFSE: I < II < IV < III.

JEE Main · 2025 · 29 Jan 2025 · Shift 1 · Single correct option