An aqueous solution of ${\mathrm{NiCl}}_2$ was heated with excess sodium cyanide in presence of strong oxidizing agent to form ${\left[\mathrm{Ni}(\mathrm{CN}{)}_6\right]}^{2-}$. The total change in number of unpaired electrons on metal centre is

An aqueous solution of ${\mathrm{NiCl}}_2$ was heated with excess sodium cyanide in presence of strong oxidizing agent to form ${\left[\mathrm{Ni}(\mathrm{CN}{)}_6\right]}^{2-}$.

${\left[\mathrm{Ni}(\mathrm{CN}{)}_6\right]}^{2-}$${\mathrm{Ni}}^{+4}\to {\mathrm{d}}^6$ Due to strong field ligand Pairing will be there hence zero unpaired electron.

${\mathrm{NiCl}}_2\to {\mathrm{Ni}}^{2+}\to {\mathrm{d}}^8$due to weak field ligand there is no pairing hence two unpaired ${\mathrm{e}}^{-}$

Hence, the total change in number of unpaired electrons on metal centre is 2.