Structural complexity in the rare earth metallocene hydride complexes, [(C₅Me₅)₂LnH]₂

X-ray crystallographic data obtained on the metallocene hydrides, [(C5Me5)2LnH]2 (Ln = Gd, Tb, and Dy), of interest for their magnetic properties, have revealed unexpected structural variability in a closely related series of rare earth complexes that can complicate magnetic analysis. Crystals of the two larger metals, Gd and Tb, were structurally straightforward and isomorphous with crystals of [(C5Me5)2SmH]2. However, only for Tb were the locations of the hydride ligands in this structural type identified for the first time and found to be consistent with a (C5Me5)2Ln(μ-H)2Ln(C5Me5)2 structure. In contrast, for Ln = Dy, the [(C5Me5)2H](3-) ligand set does not appear to have one optimum crystal structure. Two different types of crystals and one other solid form of [(C5Me5)2DyH]2 were repeatedly isolated upon crystallization and demonstrated that the structure of any particular crystalline sample selected for magnetic analysis could be variable. Asymmetric structures with a single hydride bridge, (C5Me5)2Dy(μ-H)DyH(C5Me5)2, were identifiable for the two crystalline forms. This demonstrated uncertainty in structure and highlights the importance of having a coordination environment with one preferred form for magnetically interesting complexes.