According to MOT (If z is internuclear axis) The configuration of
CN−:σ1s2,σ∗1s2,σ2s2,σ∗2s2,π2px2=π2py2,σ2pz2
Bond order=2numberofelectronsinbondingorbitals−numberofelectronsinanti−bondingorbitals
Bond order =21(10−4)=3
CN− is diamagnetic due to absence of unpaired electron.