Explanation

(d)
In both $\left\lbrack Co\left( NH_{3} \right)_{6} \right\rbrack^{3 +}\ and\ \left\lbrack CoF_{6} \right\rbrack^{3 +},\ Co$ is present as $Co^{3 +}.$
Thus, the electronic configuration of Co is
_{27}Co=[Ar] $3d^{7},4s^{2}$
_{27}Co^{3+}=[Ar]$3d^{6},\ 4s^{0}$
In case of $\left\lbrack Co\left( NH_{3} \right)_{6} \right\rbrack^{3 +},\ NH_{3}$ is a strong field ligand, so pairing of electrons in 3d-orbital takes place.
_{27}Co^{3+}=[Ar]$3d^{6},\ 4s^{0}$
[image:9_image249.emf]
In $\left\lbrack CoF_{6} \right\rbrack^{3 +},\ F$ is a weak field ligand, thus doesn’t cause pairing. Hence,
_{27}Co^{3+}=[Ar]$3d^{6},\ 4s^{0}$
[image:9_image250.emf]