Question

Arrange the following in order of decreasing number of unpaired electrons:
$\left(I\right)\left[Fe\right(H_{2}O{)}_6{]}^{2+}\left(II\right)\left[Fe\right(CN{)}_6{]}^{3-}$
$\left(III\right)\left[Fe\right(CN{)}_6{]}^{4-}\left(IV\right)\left[Fe\right(H_{2}O{)}_6{]}^{3+}$

• A. $\left(IV\right),\left(I\right),\left(II\right),\left(III\right)$
• B. $\left(I\right),\left(II\right),\left(III\right),\left(IV\right)$
• C. $\left(III\right),\left(II\right),\left(I\right),\left(IV\right)$
• D. $\left(II\right),\left(III\right),\left(I\right),\left(IV\right)$

Answer 1

The correct option is A $\left(IV\right),\left(I\right),\left(II\right),\left(III\right)$

($I):[Fe(H_{2}O{)}_6{]}^{2+}$ in this complex $H_{2}O$ is weak field ligand hence pairing will not happen.
$F{e}^{2+}:\left[Ar\right]3d^{6}4s^0=t_{2g}^4{e}_g^2$
$\therefore$ Contains 4 unpaired electrons.
($II):[Fe(CN{)}_6{]}^{3-}$ in this complex $CN$ is strong field ligand hence pairing will happen.
$F{e}^{3+}:\left[Ar\right]3d^{5}4s^0=t_{2g}^5{e}_g^0$
$\therefore$ Contains 1 unpaired electron.
($III):[Fe(CN{)}_6{]}^{4-}$ in this complex $CN$ is strong field ligand hence pairing will happen.
$F{e}^{2+}:\left[Ar\right]3d^{6}4s^0=t_{2g}^6{e}_g^0$
$\therefore$ Contains 0 unpaired electron.
($IV):[Fe(H_{2}O{)}_6{]}^{3+}$ in this complex $H_{2}O$ is weak field ligand hence pairing will not happen.
$F{e}^{3+}:\left[Ar\right]3d^{5}4s^0=t_{2g}^3{e}_g^2$
$\therefore$ Contains 5 unpaired electrons.