Question

CFSE value of $d^6$ octahedral complex at weak field is $-x{\Delta }_o$. x is

Answer 1

The configuration of $d^6$ ocathedral complex at weak field is $\left(t_{2g}{)}^4\right(e_g{)}^2$

$\text{CFSE for octahedral complex}=[-0.4(n)t_{2g}+0.6(n^{\prime }\left)e_g\right]{\Delta }_o+n\ast P$ ​
Where, ​
n: Number of $e^{-}$ occupying $t_{2g}$ orbital ​
n': Number of $e^{-}$ occupying $e_g$ orbital​
${\Delta }_o$: Ligand field splitting energy​
$n^{\ast }$: Number of extra electron pairs ​
P: Spin-pairing energy​

$CFSE=[-0.4\times 4+0.6\times 2]{\Delta }_o$
$CFSE=-1.6+1.2=-0.4{\Delta }_o$