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In valence bond theory, resonance plays a pivoting role; why isn't such concept needed in MO theory? Why is it told that "MO theory provides a global, delocalized perspective on chemical bonding"?

(This is a follow up to What is actually the difference between valence bond theory and molecular orbital theory?)

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Because MO is closer to reality.

Resonance is a convenient fiction used to simplify the visualization of some molecules, and rationalize their actions in the frame work of Lewis structures. But every resonance structure you have ever seen in your life is a lie.

Well, this is not exactly true. It's more precise to say that resonance structures are like blind men trying to describe an elephant: they each give a partial snapshot of truth while being wholly inadequate to describing it all. The "true" structure is a weighted average of all the resonance structures based on how stable they would be if they were the actual molecule.

MO cuts to the chase and attempts to directly describe the molecule in terms of its molecular orbitals: another convenient fiction, mind, but one that is closer to the truth. The reason you don't need "resonance" MO structures is because molecular orbitals are inherently a probabilistic construct: they already take into account the uncertainty in the electrons' positions.

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    $\begingroup$ "The reason you don't need "resonance" MO structures is because molecular orbitals are inherently a probabilistic construct: they already take into account the uncertainty in the electrons' positions."- what do you mean by that, sir? Atomic orbitals are themselves wavefunctions; they are also probabilistic construct & resonance is the superposition of wavefunctions of different electron arrangements for same nuclear frame; it is also a probabilistic construct! What do you actually want to point out among them saying "probabilistic construct"?? $\endgroup$ – user5764 Jul 14 '15 at 18:55
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    $\begingroup$ Let me make myself plainer. An atomic orbital is probabilistic, but only takes into account the possibilities of an electron being in one atom space. In pure valence bond theory, either the wave function for the electron is over in one atom, or the other. The issue is that the electrons are in fact delocalized. In VBing, the resonance structures are what show this delocalization. However, in MO theory, the delocalization is inherent in the construction of the molecular orbitals: the wave functions are already distributed across all the atoms a given electron interacts with. $\endgroup$ – Breaking Bioinformatics Jul 23 '15 at 16:21
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In MO theory, the molecule is described by Molecular Orbitals that are approximately calculated from atomic orbitals; I believe the process is referred to as constructing "Linear combinations of Atomic Orbitals" (LCAO).

My understanding is that you use "weighting coefficients" to say how much a particular atomic orbital contributes to a given Molecular Orbital.You also always get out as many MO's as AO's you put in.

Take for example, Benzene. If you start adding up the atomic orbitals it would be understandable to assign each atomic orbital equal weight in the calculation. So in the end you get 6 equivalent MO's.

I would like to hear someone else's perspective on this, but I'm guessing that you sort of "infer" a structure similar to that obtained from resonance because of this equivalency.

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