The molecular orbital diagram for $\ce{O2}$ says that the sigma 2p bonding molecular orbital is lower in energy than the pi 2p bonding molecular orbital. Why is this not the case in the $\ce{B2}$ MO diagram?

  • $\begingroup$ It is, though, isn't it? $\endgroup$ Oct 25 '13 at 14:48

This topic is really fun. You see in case of boron molecule $\sigma_{2s}$ and $\sigma_{2p_z}$ (being on the intermolecular axis) will have the same symmetry and hence the orbitals will overlap and result in the formation of 2 bonded $sp_z$ orbitals. So the $\sigma_{2p_z}$ orbital will obtain somewhat $s$ character and it will become unstable or in other words will be of higher energy. The $\sigma_{2s}$ MO will obtain somewhat $p$ character. So in a energy level MO diagram $\sigma_{2p}$ bonding orbital is placed above the $\pi_{ 2p}$ orbital.

However, in case of $\ce{O2}$, due to the higher nuclear charge of oxygen, the energy difference of $2s$ and $2p$ is large so there is no such kind of intermixing between the bonded MOs. Whereas in boron, the energy difference of $2s$ and $2p$ is less, facilitating intermixing.

You can find this topic in any higher level chem books. Hope you got my point; thank you.

  • 1
    $\begingroup$ Could you provide an example or link to a book that contains this information? $\endgroup$
    – Ben Norris
    Jan 15 '14 at 16:53
  • $\begingroup$ google.co.in/… $\endgroup$
    – user4179
    Jan 16 '14 at 6:49
  • $\begingroup$ Boron radius - 80 pm, Oxygen - 65 pm. Funny-funny-funny $\endgroup$
    – permeakra
    Oct 26 '14 at 9:32
  • $\begingroup$ exactly oxygen is not bigger in size.... $\endgroup$
    – oshhh
    Sep 14 '16 at 10:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.