Identifying Alkyl Ligands as Pi Donors or Pi Acceptors in Transition Metal Complexes

Just wondering if there was a systematic way to identify an alkyl ligand as a $\pi$ acceptor or $\pi$ donor in a transition metal complex? 


Although various sources indicate that weak field ligands are typically $\pi$ donors and strong field ligands are $\pi$ acceptors, I'm having trouble with the more uncommon ligands not listed in the spectrochemical species, namely $\ce{CH2}$ groups directly double-bonded to metals, or $\ce{CH2-tBu}$ groups double bonded to ligands.


