Difference between Valence bond theory and molecular orbital theory?

[mahela007]

I am having trouble identifying the difference between VB theory and molecular orbital theory. To me, they seem to be one and the same.
In VB theory, two atomic orbitals overlap and share electrons forming a bond... Wouldn't this combination of two Atomic orbitals make a molecular orbital?

 

[PhaseShifter]

They actually have different predictions in some cases.

For instance, valence bond theory predicts no unpaired electrons in molecular oxygen, while molecular orbital theory predicts two unpaired electrons in the 2\pi^{*} orbitals.

Molecular orbital theory can be verified in this case, since molecular oxygen is paramagnetic.

VB theory is reasonably good at providing qualitative information about the shapes of molecules, but MO theory provides more information about energy levels, bond strengths, etc.

 

[mahela007]

I know they ARE different.. i just don't understand how they are different according to their definitions.
As I said in the first post, VB theory says orbitals just overlap. Doesn't this overlap form a molecular orbital (i.e an orbital common to both atoms)

 

[PhaseShifter]

VB doesn't even really assume proper orbitals--it just assumes the electrons form pairs (either as bonds or nonbonding pairs) and then attempts to maximize the distance between electron pairs on a given atom based on electrostatic repulsion of electrons.

MO theory actually keeps track of energy levels, symmetry matching required for orbital overlap, bonds with multiple centers, etc.

VB theory looks at sp^{2} hybridization as merely three electron pairs spaced as far apart as they can possibly be while surrounding a central atom. MO theory looks at the same hybrid orbitals as a set of linearly independent combinations of one S orbital and two P orbitals.

Also, try modeling the 3-centered bonds that hold a BH_{3} dimer together with VB theory and you'll see it doesn't work so well.

 

[DrDu]

Loosely speaking valence bond theory describes the molecule with a linear combination of products of atomic orbitals. In MO theory, one forms first linear combinations of atomic orbitals (the "molecular orbitals") and then forms products of these. Usually, VB performs much better than MO for nonmetallic compounds(e.g. it gives a working description of Fluorine F2, while this molecule would not be stable in MO theory without further corrections), while it quickly becomes infeasible for compounds with delocalized bonding.
By the way, there is a program out there called VB2000 with which you can perform VB calculations online: http://www.scinetec.com/
Just try it out with your favourite molecule and compare with your preferred MO program.