Configuration of C2 σ1s2σ*1s2σ2s2σ*2s2π2px2=π2py2 Configuration of C2− σls2σ*1s2σ2s2σ*2s2π2px2=π2py2σ2pz1 Bond order =
No.of bounding e−−No.of antibonding e−
2
C2 has s−p mixing and the HOMO is π2px=π2py and LUMO is σ2pz. So, the extra electron will occupy bonding molecular orbital and this will lead to increase in bond order. So, C2− has more bond order than C2.