The CFSE value of an octahedral complex is −0.6 ${∆}_{\mathrm{o}}$. If metal ion is surrounded by weak ligands, the magnetic moment can be (spin only)

Given CFSE value obtained when there are four unpaired electrons ( 3 in t_2g and 1 in e_g).

$\mathrm{CFSE}=-0.4\mathrm{n}{∆}_{\circ }+0.6\mathrm{n}{∆}_{\circ }=\left(-0.4\times 3+0.6\times 1\right){∆}_{\circ }=-0.6{∆}_{\circ }$

magnetic moment $\left(\mu \right)$:

$\mu =\sqrt{\mathrm{n}(\mathrm{n}+2)}=\sqrt{4(4+2)}=4.9 \mathrm{BM}$