Which of the following complexes is expected to be diamagnetic?

Solution

1. In $[\text{Co}({\text{NH}}_3{)}_6{]}^{3+}$, the oxidation state of $\text{Co}$ is $+3$, with a $d^6$ configuration.
2. ${\text{NH}}_3$ is a strong field ligand, forcing all electrons to pair in the $t_{2g}$ orbitals.
3. The resulting configuration is $t_{2g}^6{e}_g^0$, which has no unpaired electrons.
4. Therefore, the complex is diamagnetic.
5. Hence the answer is (B).