[chimiste]

It's about the pi backbonding in metal complexes. I found the two statements contradicting and I'm confused. One says that as sigma donation of the ligand to the metal increases, the backbonding increases while the other one seems to say the opposite. 


"A sigma-type donation from the C=C pi orbital with concomitant pi-backbonding into an empty pi* orbital on the ethylene presents us with a synergistic bonding situation: the greater the sigma donation to the metal, the greater the pi-backbonding."

"the presence of electron-withdrawing groups on the olefin results in poorer sigma donation and lowers the energy of the pi* orbital (providing better overlap for backbonding)."

[TheBigF]

For pi-bound metal alkene systems there is sigma donation from the C=C pi-bond to an appropriate orbital on the metal. This increases the electron density on the metal centre so increases the likelihood of backbonding from the metal d-orbitals into the pi* (anti-bonding) C=C orbitals. {For diagrams see http://en.wikipedia.org/wiki/Pi-complex}

So the increased sigma donation does enhance pi-backbonding.

The statement about electronegative substituents is also correct, in the limit F2C=CF2 looks more like a metallo-cyclopropane in Xray structures, ie "bonds" from the M to both C's of the C2F4. This effect is also supported by spectroscopy, the more electronegative alkenes have a higher barrier to rotation.

[hmx9123]

There is a great description on backbonding in Crabtree's book on Organometallic Chemistry, if you want more information.