Which one of the following species

Solution:

$\mathrm{Fe}^{3+}:[\mathrm{Ar}] 3 \mathrm{~d}^{5}$

Hybridisation : $\mathrm{sp}^{3} \mathrm{~d}^{2}$

Magnetic nature : Paramagnetic (so this complex response to external magnetic field)

2. $\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}$

$\mathrm{Ni}^{2+}:[\mathrm{Ar}] 3 \mathrm{~d}^{8}$

Hybridisation : dsp $^{2}$

Magnetic nature : diamagnetic

3. $\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}$

$\mathrm{Co}^{3+}:[\mathrm{Ar}] 3 \mathrm{~d}^{6}$

Hybridisation : $d^{2} s p^{3}$

Magnetic nature : diamagnetic

$\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]$

$\mathrm{Ni}:[\mathrm{Ar}] 3 \mathrm{~d}^{8} 4 \mathrm{~s}^{2}$

Hybridisation : $\mathrm{sp}^{3}$

Magnetic nature : diamagnetic