According to molecular orbital theory, the species among the following that does not exist is:

🧠 A species "does not exist" if its bond order = 0 (no net bonding). Calculate BO for each.

| Species | Electrons | BO | |---|---|---| | $\ce{He2-}$ | 5 | (3−2)/2 = 0.5 — exists (barely) | | $\ce{O2^{2-}}$ | 18 | (10−8)/2 = 1 — exists ✓ | | $\ce{He2+}$ | 3 | (2−1)/2 = 0.5 — exists ✓ | | $\ce{Be2}$ | 8 | (4−4)/2 = 0 — does not exist |

$\ce{Be2}$: MO filling \sigma_{1s}^2\sigma^*_{1s}^2\sigma_{2s}^2\sigma^*_{2s}^2 — bonding and antibonding contributions cancel completely → BO = 0 → molecule cannot exist.

$\boxed{\text{Answer: (d) } \ce{Be2}}$