τQTM / s 10 2.54(7) 10 2.13 --10 1.3(2) 10 -0.36(2) 10 2.12(5) 12.2% solid angle in the SIP to 29.3% for 8-Dy (Table ). To the best of our knowledge, for the solvent-free SIP and CIP. We therefore posit that differences in SMM properties between these complexes are mainly a consequence of transverse interactions. The structural differences observed in the {Dy(Cp ttt )(Cp*)} core for complex 5-Dy have been attributed to the more sterically demanding, bidentate κ 2 -interaction of o-difluorobenzene (binding site comprises 21.1% solid angle), leading to increased Dy … Cp R centroid distances and a more acute It is unusual for a phonon bottleneck to be observed in zero field where direct singlephonon relaxation within the ground doublet does not occur, but this appears to be the case for 2-Dy, 3-Dy and 4-Dy, leading to extremely slow relaxation at low temperature for 2-Dy and 3-Dy. Phonon bottlenecks for direct single-phonon relaxation processes can have temperature exponents of m = 1-6 depending on heat conduction. A value of m = 6 is expected for a spectral bottleneck with anharmonic effects, while a value of m = 2 is characteristic of either a spatial bottleneck (mean free path of phonons are much smaller than crystal dimensions) or a two-phonon spectral bottleneck. The temperature exponents of 2.0(8) for 4-Dy and m = 4.5 (7) for 2-Dy are within the expected range for phonon bottlenecks and are consistent with these rates being highly sensitive to polymorphism, even in the presence of similar coordination environments.