where $n=$ number of unpaired electron.

Number of unpaired $e^{-} s$ and corresponding magnetic moments

$1 \rightarrow 1.7$

$2 \rightarrow 2.8$

$3 \rightarrow 3.9$

$4 \rightarrow 4.9$

$5 \rightarrow 5.9$

since $B r^{-}$ is a weak feild ligand, so electrons in $C o^{2+}$ will not pair up.

the complex has $3$ unpaired electrons leaving it paramagnetic in nature.