C2⇒σ1s2<σ∗1s2<σ2s2<σ∗2s2< HOMO π2ps2=π2py2< LUMO σ2pz0
All electrons are paired. So, the compound is diamagnetic. C2 as well as C22− is diamagnetic. More over, both bonds in C2 will be pi. In C22−, there will be one sigma bond in addition to two pi bonds.