[CrimpJiggler]

I used to dislike MO theory a lot because I thought it was some abstract, arbitary pile of purely theoretical nonsense but from encountering situations in which valence bond theory doesn't work, I'm coming around. One situation that I like to use MO theory to think about is backbonding in coordination complexes. If I'm not mistaken, when carbon monoxide bonds to an electron rich metal as a ligand, the C=O bond length increases indicating that the bond is weakened whereas if it bonds to a metal that is incapable of back bonding, the C=O bond length doesn't change. The only explanation I know of for this is that the electrons delocalised by the C=O ligands antibonding orbital weaken the bond. Is MO theory the only contemporary model that can explain this bond weakening caused by back bonding?

[obsidianavenger]

i don't know that it is "proof" so much as a confirmation that our MO theory is empirically adequate... it was obviously proposed for a reason, and this example is one indicates its superiority over other models... but its still pretty ad hoc if you ask me

[CrimpJiggler]

Yeah I agree with there 100%. MO theory seems to me to be a desperate attempt to overcome the limitations of valence bond theory. This antibonding orbital concept is very counterintuitive for me. I'm reading an organic chem book (Clayden) right now and they used to MO theory to explain carbonyl reactivity. They said that carbonyl compounds have low energy pi antibonding orbitals and nucleophiles add their electrons to this pi antibonding orbital which causes the C=O pi bond to break. This can be just as easily explained by saying that carbonyl carbon now has all the electrons it needs to complete its octet and thus no longer needs to share a pi bond with the oxygen. I could also explain the CO ligand example by assuming that the electron rich metal atom partially reduces the C=O ligand by donating some charge from its d orbitals to the carbon atoms partially emptied (due to them being shifted towards the more electronegative oxygen) p orbital thereby reducing the electrophilicity of the carbon and thus its attraction to the oxygen atom. I just explained the situation that I thought could only be explained with MO theory, using valence bond theory lol. Well thats not true, I also theres another scenario that comes to mind: the sigma bond weakening that happens to H2 ligands when they bond to a metal. I can't remember if this happens exclusively with electron rich (back bonding) metals or not though so I'm not gonna try to apply VB theory to it.  Hopefully MO theory is a step in the right direction and not a step into a bear trap like the theory of phlogiston was for 17th century chemists.

[juanrga]

I used to dislike MO theory a lot because I thought it was some abstract, arbitary pile of purely theoretical nonsense but from encountering situations in which valence bond theory doesn't work, I'm coming around. One situation that I like to use MO theory to think about is backbonding in coordination complexes. If I'm not mistaken, when carbon monoxide bonds to an electron rich metal as a ligand, the C=O bond length increases indicating that the bond is weakened whereas if it bonds to a metal that is incapable of back bonding, the C=O bond length doesn't change. The only explanation I know of for this is that the electrons delocalised by the C=O ligands antibonding orbital weaken the bond. Is MO theory the only contemporary model that can explain this bond weakening caused by back bonding?

I believed that MO theory had become accepted as a valid and useful theory by early 30s, and that it is the main theoretical basis behind computational chemistry.