In case of $\ce{[Ni(NH3)6]^2+}$, the configuration is $3\text d^8$. After splitting from spherical to octahedral field, the lower energy orbitals, i.e., $\text t_{2g}$ are already filled. Therefore, pairing of $\ce{e-}$ will not occur whether the ligand is strong or weak. So, the compound is a high-spin complex.
$\uparrow$ $\downarrow$ |
$\uparrow$ $\downarrow$ |
$\uparrow$ $\downarrow$ |
$\uparrow$ |
$\uparrow$ |
${|<------------\text t_{2g}---------->}|{<----\text e_g----->|}$
In case of $\ce{[Co(NH3)6]^3+}$, the configuration is $3\text d^6$. After splitting from spherical to octahedral field, the lower energy orbitals, i.e., $\text t_{2g}$ are not fully occupied. Therefore, pairing of $\ce{e-}$ can occur depending on the ligand. In this case, the compound is a low-spin complex.
$\text {CFSE}_\text {high spin}=-\frac{2}{5}\times 6 +\frac{3}{5}\times 2 + 0\cdot P$
Before pairing:
$\uparrow$ $\downarrow$ |
$\uparrow$ |
$\uparrow$ |
$\uparrow$ |
$\uparrow$ |
${|<-----------\text t_{2g}--------->}|{<-----\text e_g------->|}$
After pairing:
$\uparrow$ $\downarrow$ |
$\uparrow$ $\downarrow$ |
$\uparrow$ $\downarrow$ |
$~~$ |
$~~$ |
${|<-----------\text t_{2g}----------->}|{<----\text e_g----->|}$
$\text {CFSE}_\text {low spin}=-\frac{2}{5}\times 6 +\frac{3}{5}\times 0 + 2\cdot P$