Homoleptic Phosphoraneiminato Complexes of Rare Earth Elements as Initiators for Ring-Opening Polymerization of Lactones

As cyclopentadienide equivalents, the first homoleptic phosphoraneiminato complexes of rare earth elements with the very small coordination number four (see, for example, the ytterbium compound shown) are excellent initiators for the ring-opening polymerization of lactones.