Spin only magnetic moment of an octahedral complex of Fe²⁺ in the presence of a strong field ligand in BM is

$$\begin{gathered} In presence of strong firld ligand ,{∆}_o>P \\ for F{e}^{2+}=\left[Ar\right]3d^6 \end{gathered}$$

Number of unpainred electrons = 0 

Magnetic moment = 0 

In NiCl₂  

$N{i}^{2+}=\left[Ar\right] 3d^8$

Number of unpaired electrons = 2

After formation of oxidized product [Ni(CN)₆]^-2 , Ni⁴⁺ is formed

$N{i}^{4+}=\left[Ar\right] 3d^6$

CN^-  is strong field ligand. 

Number of unpaired electron = 0

The charge is 2-0=2