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.