The Relevance of Non‐Axiality and Low‐Lying Excited States for Slow Magnetic Relaxation in Pentagonal‐Bipyramidal Erbium(III) Complexes Probed by High‐frequency EPR

High-frequency/high-field electron paramagnetic resonance studies on a series of seven-coordinate pentagonal-bipyramidal (PBP) erbium(III) complexes Er(DAPMBH/H₂DAPS)X (H₂DAPMBH = 2,6-diacetylpyridine bis-4-methoxy benzoylhydrazone, H₄DAPS = 2,6-diacetylpyridine bis-(salicylhydrazone)) demonstrate the effects of different apical ligands (X = (H₂O)Cl (1), (CH₃OH)N₃ (2), Cl₂ (3)) on the local magnetic anisotropy of the central Er^III ions. In particular, we report direct experimental determination of the effective g-values and zero field splittings of the energetically low-lying Kramers doublets. Our quantitative determination of the magnetic anisotropy highlights the relevance of an axial g-tensor for SMM behavior and suggests that fast magnetic relaxation is mainly driven by a thermally assisted quantum tunnelling process via low-lying excited states.