Rethinking ytterbium(III)-based single-molecule magnets - why trigonal planar doesn't work

Yb(III)-based single-molecule magnets (SMMs) have remained conspicuously absent under zero-field conditions, appearing only with applied dc fields or under diamagnetic dilution. Utilising both CASSCF and CASPT2 methodologies, we systematically investigate the crystal field splitting of Yb(III) in a series of symmetry-constrained model complexes to elucidate the electronic origins of this limitation. By analysing equatorial ligand field environments that promote maximal axial anisotropy and benchmarking against the behaviour of Er(III), we identify fundamental differences in their electronic structures that disfavour slow magnetic relaxation in Yb(III). Quantitative estimates of the quantum tunnelling of magnetisation (QTM) rates reveal that substantial transverse g-tensor components are intrinsic to Yb(III) in most high-symmetry coordination environments, leading to rapid zero-field relaxation. Notably, even ideal D_3h, D_4h, and D_6h symmetries fail to sufficiently suppress transverse anisotropy. Only D_5h and D_4d symmetries lead to the plausible prediction of SMM behaviour owing to the formal quenching of wavefunction mixing and thus QTM. Comparatively, Er(III) complexes systematically show higher g-tensor axiality, rationalising their superior zero-field performance. Using ideal crystal field parameterisation within a model Hamiltonian framework, we establish stringent symmetry criteria specifically for Yb(III) thereby guiding future synthetic efforts.