The configuration of

$\mathrm{CN}^{-}: \sigma_{1 \mathrm{s}}^{2}, \sigma_{1 \mathrm{s}}^{* 2}, \sigma_{2 \mathrm{s}}^{2}, \sigma_{2 \mathrm{s}}^{*2}, \pi_{2 \mathrm{p}_{\mathrm{x}}}^{2}=\pi_{2 \mathrm{p}_{\mathrm{y}}}^{2}, \sigma_{2 \mathrm{p}_{\mathrm{z}}}^{2}$

Bond order $=\frac{1}{2}(10-4)$

$=3$

$\mathrm{CN}^{-}$ is diamagnetic due to absence of unpaired electron