Assertion : $\left[\mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_5 \mathrm{NO}\right] \mathrm{SO}_4$ is paramagnetic.Reason : The Fe in $\left[\mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_5 \mathrm{NO}\right] \mathrm{NO}_4$ has three unpaired electrons.
both assertion and reason are true and reason is the correct explanation of assertion
both assertion and reason are true but reason is not the correct explanation of assertion
assertion is true but reason is false
both assertion and reason are false.
Solution
(a) : $\mathrm{Fe}^{+}:[\mathrm{Ar}] 3 d^6 4 s^1$
When the weak field ligand $\mathrm{H}_2 \mathrm{O}$ and strong field ligand NO attack, the configuration changes as follows:
$\mathrm{Fe}^{+}:[\mathrm{Ar}] 3 d^7 4 s^0$
$\therefore \quad \mathrm{Fe}^{+}$has 3 unpaired electrons.
