Suppose that the ion is excited by light, so that an electron moves from a lower-energy to a higher-energy molecular orbital. would you expect the excited-state h−2 ion to be stable?

As can be seen from the 1st MO diagram on the website the overlap of the H1s AO on one H atom with the H1s AO on another H atom results in a Ď MO at lower energy to the constituent AOs, and a Ď* MO at higher energy than the starting AOs. Each MO can hold two eâ»s so the H2^- has the configuration Ď(2eâ») Ď*(1eâ») or Ď(↑↓) Ď*(↑) Bond Order = ½[ÎŁ (bonding eâ») - ÎŁ (antibonding eâ»)] bo = ½[ÎŁ (2eâ») - ÎŁ (1 eâ»)] = 0.5 the H2^- is predicted to be bound. The lowest energy electronic transition is Ď â†’ Ď*: Ď(↑↓) Ď*(↑) → hν → Ď((↑)) Ď*(↑↓) This guy has a bond order of -0.5 and hence is unbound.

It's not the same dearthe first one is in grams per mole while the other is not