All previously reported examples of axial Dy SMMs with no equatorial donor ligands contain bulky η 5cyclopentadienyl ligands or related derivatives, hence there is no literature precedent for refined magnetostructural comparisons with 1-Dy. The Dy(III) ion in 1-Dy is bound by two monodentate σ-donor bis(silyl)amides; although some of the charge density is delocalized about the ligand scaffolds due to negative hyperconjugation with the triisopropylsilyl groups, the N atoms can be formally treated as point charge Lewis bases to a first approximation. This differs from the highest-performing Dy SMMs bound by π-aromatic ligands in the literature, which donate electron density to Dy from delocalized molecular orbitals located about a pentagonal arrangement of atoms. Accordingly, the degree of magnetic anisotropy and the purity of mJ states should be more sensitive to deviations from ideal linearity in complexes like 1-Dy than for a sandwich-type complex. We previously reported the solid-state structures of the bent Tm(III) complex [Tm{N(Si i Pr3)2}2][B(C6F5)4] (N-Tm-N: 125.49(9)°), and considering that the six-coordinate ionic radii of Dy(III) (0.912 Å) and Tm(III) (0.88 Å) are quite similar, we anticipated a similar N-Dy-N angle for 1-Dy. However, previous computational studies on a series of model twocoordinate DyL2 compounds (L = mono-or di-anionic monodentate C-or N-donor ligand) predicted that Ueff should decrease regularly with bending (e.g. [Dy{N(SiH3)2}2]; Ueff = 2072 cm -1 at 180° N-Dy-N, and 919 cm -1 at 120°), Conventional usage of the average transition matrix elements of magnetic moment to infer magnetic relaxation probabilities suggests that the Ueff value for 1-Dy should be near the top of the CF manifold (Figure ), whilst the experimentallydetermined Ueff value lies in between the first and second excited state. This discrepancy highlights that the static electronic structure alone is insufficient to predict relaxation dynamics. While this would be an excellent opportunity to deploy our recent methods to calculate magnetic relaxation dynamics ab initio, unfortunately for 1-Dy there are eight formula units in the crystallographic unit cell (1472 atoms), thus this system is currently too large to perform periodic DFT calculations. We propose that the modulation of the CF by phonons (i.e. spin-phonon coupling) has a larger impact on the electronic states in 1-Dy than for [Dy(Cp R )2] + , because in the former complex the CF is almost exclusively dominated by two flexible monoatomic donor ligands rather than the more rigid  5 -Cp R rings in [Dy(Cp R )2] + and related derivatives, which have been shown to be crucial for dictating spin dynamics in [Dy(Cp ttt )2] + . 57