
#1
Nov1713, 06:22 PM

P: 149

Are there any things that can be explained by valence bond theory, that cannot be explained by molecular orbital theory? I know of various things that can only be explained by MO theory, and not VB theory, but does it go both ways?




#2
Nov1813, 01:58 AM

Sci Advisor
P: 3,378

Yes, e.g. VB yields a molecule F2 while it isn't stable in Hartree Fock theory.
I don't think there are many things that can't be explained in VB theory but only in MO theory. However, VB theory becomes cumbersome when you use it in metallic or semimetallic componds like Boranes as you need an excessive number of resonance structures. Generally, VB calculations are much more accurate than Hartree Fock calculations but also much more costly in terms of computing power. 



#3
Nov1813, 08:53 AM

P: 149

One case that I know of that can be explained only by MO theory, are back bonding in carbonyl metal complexes (the C=O bond gets weakened by electron donation from the metal, AFAIK this can only be explained by considering antibonding orbitals). Another example is cycloadditions, i.e. you can explain why photoexcited 2 + 2 cycloadditions work, by considering the phases of the MOs. Maybe these things can be explained by VB theory too, I haven't put too much thought into it.




#4
Nov1813, 10:01 AM

Sci Advisor
P: 3,378

Are there any advantanges to VB theory over MO theory?
Maybe this article is an interesting read:
http://yfaat.ch.huji.ac.il/SASON/A%2...d%20Theory.pdf Also the Woodward Hoffmann rules can be explained by VB. The case of backbonding is IMHO quite simple, just consider the following two resonance structures ## \ddot{M}C=O\leftrightarrow M=C\ddot{O}## hence the second structure will lead to a weaker bond between C and O and a stronger one between M and C. 



#5
Nov1813, 04:48 PM

P: 149

Thanks. Yeah, resonance structures explain it pretty simply alright. I didn't know resonance structures applies to coordination complexes like that. That'll help me a lot in understanding the properties of various complexes, thanks a lot.



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