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.

• Correct answer
  A

  If both assertion and reason are true and reason is the correct explanation of assertion
• B

  If both assertion and reason are true but reason is not the correct explanation of assertion
• C

  If assertion is true but reason is false
• D

  If 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.

Hence, the correct answer is option: 'A'.