Among (i) $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}$, (ii) $\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}$, (iii) $\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}$, (iv) $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{2}$ the complex which is diamagnetic is

• Correct answer
  A

  $i$
• B

  ii
• C

  iii
• D

  iv

Solution

(i) $\mathrm{Co}^{+3}=[\mathrm{Ar}] 3 \mathrm{~d}^{6} \mathrm{~s}^{0}$ $\mathrm{NH}_{3}$ is a strong field ligand
(ii) $\mathrm{Ni}^{+2}=[\mathrm{Ar}] 3 \mathrm{~d}^{8} 4 \mathrm{~s}^{0}$ $\mathrm{NH}_{3}$ is a strong field ligand
(iii) $\mathrm{Cr}^{+3}=[\mathrm{Ar}] 3 \mathrm{~d}^{3} 4 \mathrm{~s}^{0}$ $\mathrm{H}_{2} \mathrm{O}$ is a weak field ligand
(iv) $\mathrm{Fe}^{+2}=[\mathrm{Ar}] 3 \mathrm{~d}^{6} 4 \mathrm{~s}^{0}$ $\mathrm{H}_{2} \mathrm{O}$ is a weak field ligand.
So, $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}$ will be diamagnetic.

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