Which of the following complex compounds will exhibit highest paramagnetic behaviour? (At. no. : $\mathrm{Ti}=22, \mathrm{Cr}=24$, $\mathrm{Co}=27, \mathrm{Zn}=30$ )

• A

  $\left[\mathrm{Ti}\left(\mathrm{NH}_3\right)_6\right]^{3+}$
• Correct answer
  B

  $\left[\mathrm{Cr}\left(\mathrm{NH}_3\right)_6\right]^{3+}$
• C

  $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$
• D

  $\left[\mathrm{Zn}\left(\mathrm{NH}_3\right)_6\right]^{2+}$

Solution

(A) Electronic configuration of $\mathrm{Ti}^{3+}$ in $\left[\mathrm{Ti}\left(\mathrm{NH}_3\right)_6\right]^{3+}$
(B) Electronic configuration of $\mathrm{Cr}^{3+}$ in $\left[\mathrm{Cr}\left(\mathrm{NH}_3\right)_6\right]^{3+}$
(C) Electronic configuration of $\mathrm{Co}^{3+}$ in $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$ :
In the presence of strong field ligand $\mathrm{NH}_3$, pairing of electrons takes place and thus, octahedral complex, $\quad\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$ is diamagnetic. $$ \left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+} $$ $$ \left[\begin{array}{l} \text { inner orbital or } \\ \text { low spin complex } \end{array}\right]\left(6 \mathrm{NH}_3 \text { molecules }\right) $$
(D) Electronic configuration of $\mathrm{Zn}^{2+}$ in $\left[\mathrm{Zn}\left(\mathrm{NH}_3\right)_6\right]^{2+}$ :
$\therefore\left[\mathrm{Zn}\left(\mathrm{NH}_3\right)_6\right]^{2+}$ is an outer orbital complex and is diamagnetic.

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