Monophosphine-type ligands were among the first to appear in asymmetric hydrogenation, e.g., the ligand CAMP.  Continued research into these types of ligands has explored both P-alkyl and P-heteroatom bonded ligands, with P-heteroatom ligands like the phosphites and phosphoramidites generally achieving more impressive results. Structural classes of ligands that have been successful include those based on the binapthyl structure of MonoPHOS  or the spiro ring system of SiPHOS.  Notably, these monodentate ligands can be used in combination with each other to achieve a synergistic improvement in enantioselectivity; something that is not possible with the diphosphine ligands.