Unusual Complex Chemistry of Rare‐Earth Elements: Large Ionic Radii—Small Coordination Numbers

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Abstract
Because of their large ionic radii and relatively low oxidation states rare-earth elements generally form complexes which have high coordination numbers and weak metal–ligand bonds. They are often not suitable for homogeneous catalysis on account of their instability of configuration in solution. Complexes of the corresponding metal atoms with low coordination numbers may be an improvement. This type of complex can be obtained in the classical way by the introduction of bulky ligands, and recently, they were also prepared in reactions with ligand groups which offer remarkable metal–ligand bond features. This concept is demonstrated for complexes with bulky bis(trimethylsilyl)amido ligands {N(SiMe3)2}− and “slim” phosphoraneiminato ligands (NPR3−). Their suitability as catalysts for the ring-opening polymerization of lactones is reported as well.