JEE Main · 2019 · 09 Apr 2019 · Shift 1 · Chemical Bonding and Molecular Structure

Adding an electron stabilises a molecule only when that electron drops into a bonding level and lifts the bond order. In $\mathrm{C}{}_2$ the highest filled levels are the $\pi 2p$ pair, with the bonding $\sigma 2p$ orbital lying just above them and still empty; the extra electron in $\mathrm{C}{}_2{}^{-}$ goes into that $\sigma 2p$, raising the bond order from $2$ to $2.5$. So $\mathrm{C}{}_2$ is the molecule stabilised by picking up an electron.