Chemical Forums

Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: RaZ on January 24, 2007, 12:57:58 AM

Title: pi donation and pi-back donation
Post by: RaZ 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.

Title: Re: pi donation and pi-back donation
Post by: FeLiXe 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

Title: Re: pi donation and pi-back donation
Post by: RaZ 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??

Title: Re: pi donation and pi-back donation
Post by: FeLiXe 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

--

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