Question

Which of the following complex will be paramagnetic ?

• A. $\left[FeCl\right(CN{)}_4\left(O_2\right){]}^{4-}$
• B. $K_4\left[Fe\right(CN{)}_6]$
• C. $\left[Co\right(N{H}_3{)}_6]C{l}_3$
• D. $\left[Fe\right(CN{)}_5\left(O_2\right){]}^{5-}$

Answer 1

The correct option is A $\left[FeCl\right(CN{)}_4\left(O_2\right){]}^{4-}$

(A) $\left[FeCl\right(CN{)}_4\left(O_2\right){]}^{4-}$; here in the complex $O_2$ is peroxide ion, i.e. $O_2^{2-};C{N}^{-}$ is strong field ligand, so complex for the pairing of electrons.
$F{e}^{3+}\to 3d^5$

(B) $K_4\left[Fe\right(CN{)}_6];C{N}^{-}$ is strong field ligand so complex for the pairing of electrons.
$F{e}^{2+}\to 3d^6$ :

(C) $\left[Co\right(N{H}_3{)}_6]C{l}_3;N{H}_3$ is strong field ligand and $3d^6$ configuration has higher CFSE compelling for the pairing of electrons.
$C{o}^{3+}\to 3d^6$ :

(D) $\left[Fe\right(CN{)}_5\left(O_2\right){]}^{5-};$ here in this complex $O_2$ is peroxide ion, i.e. $O_2^{2-}$; $C{N}^{-}$ is strong field ligand so complex for the pairing of electrons.
$F{e}^{2+}\to 3d^6$ :