Solution

Co (Z = 27): $[Ar]\,3d^7\,4s^2$
In $[CoF_6]^{3-}$: $x + 6(-1) = -3 \Rightarrow x = +3$. $Co^{3+} = [Ar]\,3d^6$.
$F^-$ is a weak field ligand, so $\Delta_o$ is less than the pairing energy. The electrons occupy the orbitals singly first (high spin): $t_{2g}^4\,e_g^2$.
In $t_{2g}^4$ one orbital is doubly filled and two are singly filled; both $e_g$ orbitals are singly filled. Unpaired electrons $= 2 + 2 = 4$.
($[CoF_6]^{3-}$ is an outer orbital $sp^3d^2$ complex and is paramagnetic.)
So the number of unpaired electrons is 4.

Concept High spin d6 complex

Coordination Compounds in past papers

104 questions from this chapter have appeared across 27 exam years.