Single crystal XRD studies were performed on 1-Ln; as these complexes show similar structural features only 1-Ce is depicted in Figure (see Table for selected bond lengths and angles and Supporting Information Figures S7-S10 and Tables S1-S3 for other structures and crystallographic parameters). We provide an improved dataset for 1-Nd, which is similar to the previously reported structure but with more precise metrical parameters. Powder XRD studies performed on 1-Ln were in accord with samples showing bulk phase purities (See Supporting Information Figures S11-S20 and Table ). All 1-Ln exhibit distorted trigonal bipyramidal geometries, with three equatorial phosphide ligands adopting a "propeller" arrangement and two axial THF molecules, in common with the structures of 1-Tm 34 and 1-Nd; individual P-Ln-P and O-Ln-P angles vary by up to 11° from 120° and 90°, respectively. The O-Ln-O angles lie in a narrow range from 175.14(8)-175.68(9)°, and the Ln centers are all located < 0.1 Å from the P3 planes; there are no obvious trends in these parameters between larger and smaller Ln, thus these discrepancies can be attributed to crystal packing effects. The large deviation of the central LnP3O2 cores from ideal polyhedra is shown by their τ5 values (1-La: 0.76, 1-Ce: 0.78, 1-Pr: 0.77, 1-Nd: 0.78, 1-Sm: 0.78), where τ5 = 1 is a trigonal bipyramid and τ5 = 0 is a square based pyramid ( 𝟓 = ( -)/𝟔𝟎, where β is the largest and  the second-largest angle in the coordination sphere). As the Ln series is traversed for 1-Ln the mean Ln-P and Ln-O bond lengths decrease, consistent with the decrease in Ln(III) ionic radii with increasing atomic number; are ca. 0.1 Å longer than their 1-Ln analogues. The smaller ligand substituents and donor atoms in [Ln{N(SiMe2H)2}3(THF)2] lead to mean Ln-N distances that are approximately 0.4 Å shorter than the corresponding Ln-P bonds in 1-Ln for the same metal, which is not entirely accounted for by the sum of the Pyykkö covalent radii (Ln-N = 0.278 Å; Ln-P = 0.314 Å). The resultant saturation of the metal coordination spheres in [Ln{N(SiMe2H)2}3(THF)2] leads to their mean Ln-O distances being ca. 0.1 Å longer than those of the corresponding 1-Ln analogue.