Question

The number of unpaired electrons in the complex ion $[Co{F}_6{]}^{3-}$ is
(Atomic number Co=27)

• A. 4
• B. 0
• C. 2
• D. 3

Answer 1

The correct option is A 4

In complex ion $[Co{F}_6{]}^{3-}$, $Co$ is present at +3 oxidation state
${}_{27}Co=1s^2,2s^{2}2p^6,3s^{2}3p^{6}3d^7,4s^2$
$C{o}^{3+}=1s^2,2s^{2}2p^6,3s^{2}3p^{6}3d^6$

Electrons will not paired up because $F^{-}$ is a weak field ligand. Thus the number of unpaired electrons in 3d sub-shell of $[Co{F}_6{]}^{3-}|$ is 4.