In the 1 H NMR spectrum of 3-H, the hydride resonance is detected as a downfield singlet at  10.3 ppm while the aromatic Cp ttt protons give rise to a singlet at  6.13 ppm. Unrestricted rotation of the Cp ttt ligands leads to an overall C 2v symmetry in solution, as evidenced by the presence of two singlets for the t Bu groups in a 2:1 ratio. In the IR spectrum of 3-H, the Lu-H stretching frequency could not be identified, even upon direct comparison with the spectrum of 3-D, similarly to what was also reported for the Ce analogue [Ce(Cp ttt ) 2 H]. The molecular structures of 3-H and 3-D were unambiguously established by single-crystal XRD analyses, confirming the formation of rare examples of monomeric Ln III complexes bearing terminal hydride or deuteride ligands (Figure and Figure ). Although the H and D atoms on lutetium were located on the electron density map, the Lu-H/D separations (1.73(8) and 1.81(5) Å, respectively) suffer from low precision and may not be fully representative of the actual bond distances. They appear particularly short compared to the lutetiumhydride distances in related Cp-type complexes. The Lu-Cp(ctr) distances in 3-H/D are identical, within experimental error, to those in 2, whereas the presence of the hydride leads to a more acute Cp(ctr)-Lu-Cp(ctr) angle (in average 167.2° in 2 vs. 153.1° in 3-H), i.e. a larger deviation from linearity.