Can someone tell me why N2^2+ is a diamagnetic molecule based on MO theory and diagrams? I get a bond order of 2, which is paramagnetic.

Answer 1

N2 has pi2py pi2pz and sigma2p filled. The two electrons in sigma 2p are at a higher energy level than the pi bonds. No antibonding electrons are present in the 2p orbitals. The bond order is 3 since that is 6 electrons/2 = 3.
The two sigma 2p electrons occupy the highest energy. When the 2+ ion is made, those two electrons disappear leaving the 2+ ion. In terms of energy level, those two electrons are the outside two electrons and those are the ones to be ionized because it requires less energy to remove them and the others. Hund's rule states (for MO theory also) that electrons occupy the same energy level singly before pairing. It would take MORE energy to remove ONE of the sigma 2p electrons PLUS one from the lower pi2px or pi2pz bonding set. Removing both sigma 2p electrons leaves all the remaining electrons paired and that will be a diamagnetic molecule. We initially had 6 electrons, we now have 6-2 or 4 electrons and that is a bond order of 4/2 = 2.