Sigma and Pi Bonds for Diatomic Oxygen Molecule

[Conservation]

According to the Molecular orbital theory, diatomic oxygen should have σ_2px (internuclear axis) and $\pi$_2py and $\pi$_2pz orbitals filled with two unpaired electrons, one at antibonding $\pi$_2py and the other at antibonding $\pi$_2pz. And of course, the 2s bonding and antibonding orbitals as well.
According to the molecular orbital theory, does this imply that diatomic oxygen possesses three "bonds" and one set of unpaired electrons, opposed to the double bond (sigma pi) implied by the valence bond theory?

Thank you.

 

[DrDu]

The unpaired electrons are in fact antibonding, so that the bond order is 3-2x1/2=2.
In contrast to folk expositions of VB theory, VB theory predicts the same ground state as MO theory and not two double bonds, which rather describes bonding in the excited singulet oxygen.
This was shown already in 1937 by Wheland and Lennard-Jones:
http://pubs.rsc.org/en/content/articlelanding/1937/tf/tf9373301499#!divAbstract

 

[Conservation]

Right, hence the paramagnetic properties displayed in oxygen to point out the flaws of VB theory.

However, if the two unpaired electrons are antibonding, does that make the π2py and π2pz bonds "half" bonds? So one "full" sigma bonds at x and two "half" bonds at y and z planes to add up to two?

 

[DrDu]

Right, hence the paramagnetic properties displayed in oxygen to point out the flaws of VB theory.

As I tried to explain, VB gives a correct description of bonding in oxygen, so it is not flawed.
Furthermore, also the singlet state of oxygen is about as paramagnetic as the triplet state, due to orbital momentum, so oxygen being paramagnetic does not help to decide whether the ground state is singlet or triplet.

However, if the two unpaired electrons are antibonding, does that make the π2py and π2pz bonds "half" bonds? So one "full" sigma bonds at x and two "half" bonds at y and z planes to add up to two?

Yes, the pi bonds both have a bond order of 1/2.

 

[LudusRex]

I would like to clarify that the concept of "bonds" in molecular orbital theory is different from that in valence bond theory. In molecular orbital theory, the concept of bonds is based on the idea of electron sharing between atoms, rather than the overlapping of atomic orbitals as in valence bond theory.

In the case of diatomic oxygen, the molecular orbital theory predicts the presence of three "bonds" - two sigma bonds and one pi bond. However, these bonds are not localized between two specific atoms, as in valence bond theory. Instead, they are spread out over the entire molecule and involve the sharing of electrons between all the atoms in the molecule.

Furthermore, the presence of unpaired electrons in certain antibonding orbitals does not necessarily imply the existence of additional bonds. These electrons are actually destabilizing the molecule and make it more reactive. Therefore, we cannot equate the number of unpaired electrons with the number of bonds in molecular orbital theory.

Overall, the concept of bonds in molecular orbital theory is more complex and involves a more holistic understanding of the electronic structure of molecules. It cannot be directly compared to the simpler concept of bonds in valence bond theory.