Yeah, look up the P-X backbonding for PF3 or P(OR)3. There are some papers that quantify and rank the effect. See for example Anthony's paper on QALE: http://dx.doi.org/10.1039/A909250A
There is a simplified MO diagram at the following link. As Chip said, a low-laying sigma* MO on the phosphine matches up with a d-orbital on the metal. Similar to CO, but in CO it's a pi* MO.
Thank you! I was still finding some references invoking the d-orbital idea and wanted to check that it was no longer a real consideration. I had also come across this Chem Rev paper (Chem. Rev. 1994, 94, 1339-1374) that was interesting (although a bit older). I actually ran a quick calculation for P(CH3)3 in Spartan before discussing this in class and it did a decent job of showing the MOs that would interact as pi-acceptors - at least decent enough to convince my students. :)
Thanks for the mention, Kyle! The involvement of phosphorus d orbitals in backbonding has been pretty much thoroughly been supplanted, correctly, by interactions of the sigma* orbitals of the P-substituent bonds. Every once in a while I still see someone invoke the d orbital explanation, but they are really the outliers at this point.
Guy Orpen has a nice paper that talks about realtive amounts of backbonding by common ligands.
I think that what everyone agrees upon is the fact that phosphites and halophosphines are good pi-acceptors. There is still a bit of disagreement over the pi-acceptor abilities of alkyl and aryl phosphines. I have always thought of them as primarily sigma-donor ligands, but computational work suggests that they do have some pi-accepting ability, albeit less than the other classes of phosphorus(III) ligands. I would suggest using P(OMe)3 as your example ligand or even cage [P(OCH2)3C-Et] for one that has tied back ligands and would limit the minimization time needed for the calculation.
No. Low lying sigma* orbitals.
Yeah, look up the P-X backbonding for PF3 or P(OR)3. There are some papers that quantify and rank the effect. See for example Anthony's paper on QALE: http://dx.doi.org/10.1039/A909250A
There is a simplified MO diagram at the following link. As Chip said, a low-laying sigma* MO on the phosphine matches up with a d-orbital on the metal. Similar to CO, but in CO it's a pi* MO.
http://www.lookfordiagnosis.com/mesh_info.php?term=Phosphines&lang=4
Thank you! I was still finding some references invoking the d-orbital idea and wanted to check that it was no longer a real consideration. I had also come across this Chem Rev paper (Chem. Rev. 1994, 94, 1339-1374) that was interesting (although a bit older). I actually ran a quick calculation for P(CH3)3 in Spartan before discussing this in class and it did a decent job of showing the MOs that would interact as pi-acceptors - at least decent enough to convince my students. :)
Thanks for the mention, Kyle! The involvement of phosphorus d orbitals in backbonding has been pretty much thoroughly been supplanted, correctly, by interactions of the sigma* orbitals of the P-substituent bonds. Every once in a while I still see someone invoke the d orbital explanation, but they are really the outliers at this point.
Guy Orpen has a nice paper that talks about realtive amounts of backbonding by common ligands.
http://pubs.acs.org/doi/abs/10.1021/om061151z?journalCode=orgnd7
I think that what everyone agrees upon is the fact that phosphites and halophosphines are good pi-acceptors. There is still a bit of disagreement over the pi-acceptor abilities of alkyl and aryl phosphines. I have always thought of them as primarily sigma-donor ligands, but computational work suggests that they do have some pi-accepting ability, albeit less than the other classes of phosphorus(III) ligands. I would suggest using P(OMe)3 as your example ligand or even cage [P(OCH2)3C-Et] for one that has tied back ligands and would limit the minimization time needed for the calculation.