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-

Correct answer: (B) I lt II lt IV lt III. Neglecting pairing energy I. [Mn(CN) B]^3- ⇒ Mn^3+, t_2 ~g^4, CFSE=-0.4 × 4 Δ o =-1.6 Δ 。 II. [Co(CN) 8]^4- ⇒ Co^2+, t_2^6 eg^1, CFSE=-0.4 × 6+ 0.6 × 1=-1.8 Δ_° III. [Fe(CN)_8 ]^4- ⇒ Fe^2+, t_2 g^6 e^0, CFSE=-0.4 × 6= -2.4 Δ_0 IV. [Fe(CN)_8 ]^3- ⇒ Fe^3+, t_2 g^5 e^0, CFSE=-0.4 × 5= -2 Δ_0 Order of stability III > IV > II > I

Q20 / 75 · JEE Main 2025 (29 Jan Shift 1) · Chemistry

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. $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}$

A
$\mathrm{IV} \lt \mathrm{III} \lt \mathrm{II} \lt \mathrm{I}$
Correct answer
B
$\mathrm{I} \lt \mathrm{II} \lt \mathrm{IV} \lt $ III
C
III $ \lt $ II $ \lt $ IV $ \lt $ I
D
II $ \lt $ III $ \lt $ I $ \lt $ IV

Solution

Neglecting pairing energy I. $[\mathrm{Mn}(\mathrm{CN}) \mathrm{B}]^{3-} \Rightarrow \mathrm{Mn}^{3+}, \mathrm{t}_2 \mathrm{~g}^4, \mathrm{CFSE}=-0.4 \times 4 \Delta \mathrm{o}$ $=-1.6 \Delta$ 。 II. $[\mathrm{Co}(\mathrm{CN}) 8]^{4-} \Rightarrow \mathrm{Co}^{2+}, \mathrm{t}_2{ }^6 \mathrm{eg}^1, \mathrm{CFSE}=-0.4 \times 6+$ $0.6 \times 1=-1.8 \Delta_{\circ}$ III. $\left[\mathrm{Fe}(\mathrm{CN})_8\right]^{4-} \Rightarrow \mathrm{Fe}^{2+}, \mathrm{t}_{2 g}{ }^6 \mathrm{e}^0, \mathrm{CFSE}=-0.4 \times 6=$ $-2.4 \Delta_0$ IV. $\left[\mathrm{Fe}(\mathrm{CN})_8\right]^{3-} \Rightarrow \mathrm{Fe}^{3+}, \mathrm{t}_{2 g}{ }^5 \mathrm{e}^0, \mathrm{CFSE}=-0.4 \times 5=$ $-2 \Delta_0$ Order of stability III $>$ IV $>$ II $>$ I

Hence, the correct answer is option: 'B'.

Solution

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