$\Delta_{o}<$ pairing energy.

$\mathrm{CFSE}=(-0.4 x+0.6 y) \Delta_{o}$

where, $x$ and $y$ are no. of electrons occupying $t_{2 g}$ and es orbitals respectively.

For $\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ complex ion.

$\mathrm{Fe}^{3+}\left(3 d^{5}\right) =t_{2 \mathrm{g}}^{3} e_{\mathrm{g}}^{2}=-0.4 \times 3+0.6 \times 2$

$=0.0 \text { or } 0\; \mathrm{Dq}$