The difference in the number of unpaired electrons of a metal ion in its high-spin and low-spin octahedral…

The electronic configuration for ${\mathrm{Co}}^{2+}\left(\mathrm{z}=27\right)=4{\mathrm{s}}^{0}3{\mathrm{d}}^7\mathrm{ }$

When it combines with weak ligands (high spin complex),${\mathrm{t}}_{2\mathrm{g}}^{2,2,1}\mathrm{ }{\mathrm{eg}}^{1,1}$, it has $3$ unpaired electrons, whereas when it bonds with strong ligands (low spin complex), ${\mathrm{t}}_{2\mathrm{g}}^{2,2,2}\mathrm{ }{\mathrm{eg}}^{1,0}$, then, it has $1$ unpaired electron.

The difference in number of unpaired electrons is equal to two.