Question

Assertion (A) : $\left[ CoF _{6}\right]^{3-}$ is paramagnetic.
Reason (R) : $Co ^{3+}$ has $3 d^{6}$ outer electronic configuration. The unpaired electrons do not pair up because of weak field provided by $F ^{-}$.

• A. A and R both are correct and R is the correct explanation of A.
• B. A and R both are correct but R is not the correct explanation of A.
• C. A is incorrect while R is correct.
• D. A and R both are incorrect.

Answer 1

Answer: (A)

In complex $\left[ CoF _{6}\right]^{3-}$ oxidation number of $Co =+3$
Electronic configuration of
$Co ^{3+}=1 s^{2}, 2 s^{2}, 2 p^{6}, 3 s ^{2}, 3 p^{6}, 3 d^{6}$

In $Co ^{3+}$ number of unpaired electrons is four, is the ligand of weak field. Hence it doesn't paired up electrons of $d$-orbital.
So, In $\left[ CoF _{6}\right]^{3-}$

So, $\left[ CoF _{6}\right]^{3-}$ is paramagnetic.
$Co ^{3+}$ has $3 d^{6}$ outer electronic configuration, but four unpaired electrons don't pair up because of weak field provided by $F ^{-}$.
Hence, assertion is correct and reason is the correct explanation of assertion