Compounds combining the large rare-earth-metal (Ln) centers with the smallest anionic ligand, H À (hydrido), continue to pose challenging questions both in fundamental and applied chemistry. [1] The inherent bonding properties in solid-state binary LnH x phases (e.g., causing metallic behavior) as well as in ligand-supported molecular counterparts (revealing unique cluster chemistry, see Supporting Information) have been the focus of extensive research. Moreover, heterobimetallic solid-state materials, such as Ni 5 LaH x , feature approved rechargeable battery components or, such as LnAlH 6 (obtained from LnCl 3 and NaAlH 4 by the release of hydrogen), are discussed as intermediate-temperature hydrogen-storage materials. [2] On the other hand, the quest for soluble molecular hydrides has triggered immense research efforts. In the meantime, mono and dihydrido derivatives "L 2 LnH" and "LLnH 2 " (L = monoanionic ligand), respectively, are assigned a crucial role in a variety of stoichiometric and catalytic transformations, [3] whereas complexes of type [LnH 3 (Do) x ] (Do = neutral donor ligand) are still elusive. While mono hydride complexes can exist as monomers, e.g., [(C 5 H 2 tBu 3 ) 2 CeH], [4] dihydrido species "LLnH 2 ", carrying only one ancillary ligand per lanthanide center, tend to form polynuclear complexes (see Supporting Information) containing as few as two [5] and up to six lanthanide metal centers. [6] Several types of ancillary ligands have been employed in an effort to stabilize complexes of low nuclearity, including sterically demanding cyclopentadienyl derivatives such as C 5 Me 4 SiMe 3[6] tris(pyrazolyl)borato scorpionates, [7] tetraazacycloamido, [8] bis(phosphinophenyl)amido pincer, [5] and pyridylamido [9] ligands as well as chelating diamido ligands (see Supporting Information). [10] However, the synthesis of a monomeric rare-earth-metal dihydride was not successful to date.The group of Takats used the sterically demanding hydrotris(3-tert-butyl-5-methylpyrazolyl)borato ligand (Tp tBu,Me ) to stabilize Ln 2+ centers in species such as alkyls, [11] carbenes, [12] amides, [11b] halides, [11,13] or hydrides [14] and was also able to obtain lanthanide dihydride complexes using the less-bulky dimethyl, diisopropyl, or unsubstituted derivative of the Tp ligand, but reported the formation of a mixture of products for the more bulky Tp tBu,Me ligand
