Any inorganic chemist knows that phosphines (in general) are good sigma donors. When phosphines are involved, however, there is a lot of suspicion about pi accepting abilities of phosphines or whether the empty d orbitals are involved in bonding or not. I have to admit, it's confusing. So, here is a great article on phosphines where the authors try to answer these questions. It was published in 1992 and I did not check how many times it was cited or if there have been any corrections made.
- The most surprising result for me was that ALL phosphines ( PMe3, PH3, P(OMe)3, PF3) are GOOD sigma donors. Yes, even PF3? Who would have thought?
- When it comes to pi acidity though the trend is as follows :
PMe3 < P(OMe)3 < PF and they conclude that PF3's pi acidity is "of comparable strength to CO"
-So, you want a pi-acceptor, but not the beast? Use PF3, if you can find it in the lab.
-How about d-orbitals? Well, according to the study, d-orbitals do not directly act as pi-acceptors, instead, they are "polarization functions" that help to increase pi acidity.
Also, this article looks like a good follow-up. Maybe I'll read at too.