JEE Main 2025 Jan 23 Shift 2, Chemistry Q14: Introduction And Nomenclature

Determine the oxidation state of the central metal, then read off the $d^n$ count.

(A) $[Fe{O}_4{]}^{2-}$ : Fe is in the $+6$ state $\Rightarrow$ [Ar]$3d^2$.

(B) $[Mn(CN{)}_6{]}^{3-}$ : Mn${}^{+3}$ $\Rightarrow$ [Ar]$3d^4$. $✓$

(C) $[Fe(CN{)}_6{]}^{3-}$ : Fe${}^{+3}$ $\Rightarrow$ [Ar]$3d^5$.

(D) Chromium(II) acetate dimer : each Cr is $+2$ $\Rightarrow$ [Ar]$3d^4$. $✓$

(E) $[Ni{F}_6{]}^{2-}$ : Ni${}^{+4}$ $\Rightarrow$ [Ar]$3d^6$.

Only (B) and (D) have $d^4$ centres.