Could diatomic helium exist? [closed]








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I know that $\ce{He2}$ (homonuclear diatomic helium) does not exist because its bond order is zero. Since the bond order of $\ce{He2+} = 1/2$, that means that the positive $\ce{He2+}$ ion exists, but how does the positive ion exist if the neutral molecule doesn't?




Although we know that the helium dimer has a tiny bond dissociation energy, we don't consider it a molecule, as the "bond" does not survive a molecular vibration, even rotation of the molecule is enough to cleave it.


From an answer by Lighthart on "[Molecular orbital theory & predicting the stability of a molecule?](https://chemistry.stackexchange.com/q/4439/4945)" I took the liberty to borrow the MO schemes:


![MO schemes helium dimers](https://i.stack.imgur.com/K2Nfs.png)


In the scheme you can clearly see how we arrive at the bond order of zero for $\ce{He2}$. Both, the bonding $\sigma$ and the anti-bonding combination $\sigma^{\*}$ are doubly occupied. Dihelium is only held together by weak van der Waals forces.  

In the case of the dihelium cation $\ce{He2+}$, one electron is removed from the anti-bonding orbital. There are now more electrons in bonding than anti-bonding orbitals. This effectively results in a net energy loss or bond energy gain, and the overall bond order increases to 0.5. In a more phenomenological context, three negative charges are now in a field of four positive charges, this gives rise to Coulomb attraction.


Reference: [Wikipedia article on dihelium](https://en.wikipedia.org/wiki/Helium_dimer#Molecule) and references therein. 


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