Question

The CFSE value of $d^5$ configuration of octahedral complex in presence of strong field ligand is '$-x{\Delta }_o$'. what is the value of 'x'

Answer 1

The given complex is a octahedral complex and the ligand is a strong field ligand. Thus, it is a low spin complex, so the pairing energy will be less than the crystal field splitting energy.

$d^5$: $t_{2g}^5{e}_g^0$
$\text{CFSE for octahedral complex}=[-0.4(n)t_{2g}+0.6(n^{\prime }\left)e_g\right]{\Delta }_o+n\ast P$ ​
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​

The CFSE of a $d^5$ octahedral complex is
$=-0.4{\Delta }_o\times 5+0.6{\Delta }_o\times 0=-2.0{\Delta }_o+0=-2.0{\Delta }_o$