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Topic: pi donation and pi-back donation  (Read 5937 times)

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Offline RaZ

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pi donation and pi-back donation
« on: January 24, 2007, 12:57:58 AM »
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.

Offline FeLiXe

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Re: pi donation and pi-back donation
« Reply #1 on: January 24, 2007, 06:31:23 PM »
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
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Offline RaZ

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Re: pi donation and pi-back donation
« Reply #2 on: January 24, 2007, 07:50:22 PM »
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??

Offline FeLiXe

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Re: pi donation and pi-back donation
« Reply #3 on: January 26, 2007, 10:24:16 AM »
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
Math and alcohol don't mix, so... please, don't drink and derive!

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