Phosphine ligand
= Phosphine ligand
A <phosphine ligand> such as $\mathrm{PR_3}$ donates a phosphorus <lone pair>. It can also accept <pi backbonding> into orbitals with substantial P–R antibonding character; phosphorus $3d$ orbitals need not be essential acceptors. Alkyl substitution generally increases sigma donation relative to electron-withdrawing substituents.