Question

why H2 is more stable as compared to He2+ ion?

Answer 1

because he2+ ion has more electrons in the antibonding molecular orbital as compared to h2
and we know that no. of electrons in the anti bondind m.o. is directly proportional to instability of a molecule

Answer 2

There is a very common misconception that for a molecule both bonding and antibonding MOs have equal gap from the energy level of the individual atomic orbitals (AOs). But this not true,I will not go into the details as they are very complicated quantum mechanical calculations,but what is important is that from these calculations it has been established that the extent of destablisation caused by antibonding levels are more than the stabilisation caused by the bonding levels.

To simply put it,if you apply it for H2,it means that the gap between antibonding sigma* and individual Hydrogen 1s AOs is higher than the gap between a bonding sigma level and the 1s atomic orbital of Hydrogen.

Showing it in  diagram,

This is for  H2 molecule.Notice the gap of the sigma* level being more than that of sigmalevel.So,when you start putting electron in H2+ to form H2-,you are basically first adding 1 electron in the sigma bonding level,then another in the sigma* antibonding level.The ectra electron in sigma level will stabilise the molecule whereas the extra electron in sigma* will destabilise it.Had the gap between sigma and sigma* from the Hydrogen 1s AOs been equal,the extent of stabilisation and destabilisation woyld have been same.But,According to the discussion above,it is easy to see that 1electron in the sigma* will destabilise the molecule more than the stability gained by the additional electron in the sigma bonding level.So,as a whole,H2- will be more destabilised than H2+.