An Ab Initio Calculation and Feature Attribution Analysis Study on Bipyramidal Mononuclear Dysprosium Complexes: Magnetic Anisotropy, Single-Molecule Magnet Property, and the Effect of Covalency.

Both pentagonal bipyramidal (PB) and hexagonal bipyramidal (HB) mononuclear dysprosium (Dy) complexes are highly efficient in generating a stable single-molecule magnet (SMM) with strong axial magnetic anisotropy. Thus, they are suitable for constituting a platform for future development of stable high-performance SMMs. However, we still lack a detailed theoretical study of both PB and HB Dy complexes. To cover this shortage, we provide here a combined ab initio calculation and feature attribution analysis of 12 PB and 12 HB Dy complexes. Compared to PB complexes, HB ones usually have slower thermally activated relaxation but faster quantum tunneling of magnetization. This may lead to a lower SMM performance of HB complexes even though their effective barriers are usually higher. The importance of features, which are more sensitive to the direction rather than the distance or electric charge of ligating atoms, supports the significant role of covalency, which is more sensitive to direction than electrostatics. The effect of covalency is quantified by the change from full structures to various charge-embedded model structures. Total covalency is detrimental to SMM performance, and its effect can be huge. This comes from the fact that the covalency from equatorial ligands is destructive and quite stronger than the constructive covalency from axial ligands. The constructive nature of axial covalency comes from its destabilization of 4f and 4f orbitals of the central Dy ion. However, equatorial covalency is more efficient in destabilizing 4f and 4f orbitals, which disfavors SMM performance.