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Robert Crabtree's The Organometallic Chemistry of the Transition Metals (7e) describes a pi-donor ligand as being a pi-donor "as a result of the lone pairs that are left after one lone pair has formed the M-L sigma bond". This, of course, makes sense, and the author goes on to explain that this pi-donation is a destabilizing effect.

Later in the chapter, he goes on to call ethylene a pi-donor because it forms the primary M-L pi-bond via the donation of pi-electrons. Of course, I understand that both types of ligands described are technically donating pi-electrons, but I find it very confusing that the term pi-donor is used to describe two entierly different ligand effects -- one that is not the primary bond and destabilizes the M-L bonding interaction and one that is the actually M-L bonding interaction.

My question is wheather or not there are better terms for differentiating the primary bonding and the secondary interactions. For example, I find it confusing to call ethylene a "pi-donor and pi-acceptor ligand" since forms its primary bond through pi-donation and back bonds via pi-acceptance.

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    $\begingroup$ Personally I’d use “pi-coordination” rather than “pi-donation” for the ethylene example $\endgroup$
    – Andrew
    Jan 14, 2021 at 20:27
  • $\begingroup$ @Mithoron: I should elaborate that it is destabilizing towards the M-L bonds, not the lone pairs of the pi-donor. Is this what you meant? $\endgroup$
    – Eli Jones
    Jan 14, 2021 at 21:49
  • $\begingroup$ ** Destabilizing the M-L bonds due to electron-electron repulsions between the t2g set and the electrons in the lone pairs $\endgroup$
    – Eli Jones
    Jan 14, 2021 at 21:59
  • $\begingroup$ Huh, I guess you're right then, with this. Still, I don't exactly get your point with the question. What's wrong with one group being sigma donor / pi donor and the other pi donor / pi acceptor? $\endgroup$
    – Mithoron
    Jan 14, 2021 at 22:04
  • $\begingroup$ @Mithoron: I just thought the similarity between the two concepts was confusing and could get mixed up, but I thought Andrew's comment cleared up my concern (eg. I would rather refer to ethylene as having pi-coordination and being a pi-acceptor than saying it is a pi-donor and a pi-acceptor) $\endgroup$
    – Eli Jones
    Jan 14, 2021 at 22:11

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