Which of the following complex compounds will exhibit highes paramagnetic behaviour?

• 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

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

Solution

(a) Electronic configuration of $\mathrm{Ti}^{3+}$ in $$ \begin{array}{l} {\left[\mathrm{Ti}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}} \\ \mathrm{Ti}^{3+}=3 \mathrm{~d}^{1} \end{array} $$
(b) Electronic configuration of $\mathrm{Cr}^{3+}$ in $$ \begin{array}{l} {\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}} \\ \mathrm{Cr}^{3+}=3 \mathrm{~d}^{3} \end{array} $$
(c) Electronic configuration of $\mathrm{Co}^{3+}$ in $$ \begin{array}{l} {\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}} \\ \mathrm{Co}^{3+}=3 \mathrm{~d}^{6} \end{array} $$
In the presence of strong field ligand $\mathrm{NH}_{3}$, pairing of electrons takes place and hence, octahedral complex, $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$ is diamagnetic. $$ \begin{aligned} \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+} &\left[\begin{array}{l} \text { inner orbital or } \\ \text { low spin complex } \end{array}\right] \\ &\left(6 \mathrm{NH}_{3}\right. \text { molecles) } \end{aligned} $$
(d) Electronic configuration of $\mathrm{Zn}^{2+}$ in $[\mathrm{Zn}$ $\left.\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}$
$\therefore \quad\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}$ is an outer orbital complex and is diamagnetic.

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