Identify a molecule which does not exist.

According to Molecular Orbital Theory, a molecule is stable only if its Bond Order (B.O.) is greater than zero.For ${\text{He}}_2$: Total electrons = 4.Electronic configuration: $\sigma 1s^2,{\sigma }^{\ast }1s^2$.B.O. = $\frac{N_b-N_a}{2}=\frac{2-2}{2}=0$.Since the bond order is zero, ${\text{He}}_2$ does not exist.