[horse]

I think about chemistry pretty much entirely through VB theory, I was asked what the limitations of it are and I couldn't really give many examples. I know that in transition metal complexes, when the metal is electron dense, the ligands bonds (i.e. the C=O bond in carbonyl ligands weakens) weaken and you can't really explain that without the concept of anti bonding orbitals.

But I googled it, and saw people listing examples and I don't understand their examples. Heres some examples people said (A for answer, C for my own comment):

A1.) The tetravalency of carbon cannot be explained by VB theory
C.) Orbital hybridization between the 2s and 2p orbitals, I don't see how that violates VB theory. With the Aufbau principal, carbon should have 1 filled 2s orbital and 2 partially filled p orbitals. Does that mean is should be divalent according to VB theory? How does MO theory explain it better?

A2.) It can't predict molecular geometry. Using H2S as an example, the angle predicted should be 90 degrees, but its really 104.5.
C.) Aren't lone electron pair orbitals part of VB theory? I can in no way see how this has anything to do with MO theory.

A2.) VB theory can't explain the colors of transition metal complexes. This one I get, but not 100%.

C.) There are different mechanisms by how metal complexes absorb UV-Vis, I'll go through the ones I know.
 - d orbital degeneracy, when the d orbitals of the metal split in the presence of the ligands, 2 of the d orbitals are forced into a higher energy position. The energy difference between these degenerate sets of d-orbitals is in the UV-Vis range of energy so these metal complexes can absorb UV-Vis photons which let electrons from the lower energy d-orbitals jump to the higher energy ones. Nothing about that violates VB theory from what I can see.

However I can see that the extent to which various ligands cause this crystal field splitting between these two sets of orbitals, thats gonna be influenced by back bonding so I suppose thats one factor where VB theory can't fully explain the colours of this kind of complex. Is there more to it than that?

 - Metal-ligand charge transfers - This one confuses me a bit. Using potassium dichromate as an example, its colour is due to this kind of interaction. The potassium atoms have no empty p orbitals so VB bond theory can't explain where the electrons from the chromium atoms electron dense d-orbital could possibly go when this transfer occurs, is that right? So I'm guessing then, the only explanation is that charge jumps from some MO connected to the metal, to the MO of the bond between the metal and potassium, is that correct? The metal by itself without the ligands can't actually have a molecular orbital can it? So which molecular orbital does the charge come from?

Looking back over the examples, I see most of them are about metal-ligand complexes and in these cases I can see how VB theory fails to fully explain these things. All the examples I can see involve antibonding orbitals allowing delocalisation of electrons though. Are there more mechanisms by which transition metal complexes violate VB theory? Also when thinking of a molecular orbital, wouldn't be transition metal complex itself be considered one big molecular orbital? When thinking of MO theory, can you break it down and think about sub molecular orbitals individually? Like nitromethane for example, it will have its own overall molecular orbital, but then the nitro group could have its own MO also, and the energy of the nitro groups MO is gonna be very different to the benzene rings MO.

[AWK]

Angle in H₂S is 92.1°