[RaZ]

Hi, I have a question regarding metal complexes formed with diene, cyclobutadiene and allylic system.  I don't understand why in a diene C1-C2 bond and C3-C4 bonds lengthen due to pi-antibonding while C2-C3 bond length shortens due to pi ponding.  In cyclobutadiene, I don't believe there is bond lengthening due to pi-antibonding (something about the nodes) and I have no clue what happens to allylic system.  If someone could help me understand this it would be great.  Thanks.

[FeLiXe]

the reason is that the LUMO accepts electrons. If you draw the LUMO you will notice that it has high electron density between C2 and C3

[RaZ]

So due to pi-donation by the ligand from the HOMO, C1-C2 and C3-C4 bond would shorten while due to pi-back bonding from the metal to LUMO of ligand, the C2-C3 bond would lengthen??

[FeLiXe]

they should both work the same way

pi-donation takes density out of the HOMO, therefore lengthens C1-C2 and C3-C4 and shortens C2-C3

pi-back bonding puts density into the LUMO and does just the same

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am more qualitative approach that seems to work well too is drawing the metalla-cyclopentene resonance structure. Its only double bond is C2=C3