Probing the Axial Distortion Effect on the Magnetic Anisotropy of Octahedral Co(II) Complexes

Three mononuclear octahedral Co(II) complexes are reported, [Co(py)₄(SCN)₂] (<b>1</b>), [Co(py)₄(Cl)₂]·H₂O (<b>2</b>), and [Co(py)₄(Br)₂] (<b>3</b>), that exhibit different distortions with compression (<b>1</b>) or elongation (<b>2</b> and <b>3</b>) of the axial positions. Easy plane magnetic anisotropy was confirmed by magnetic, HF-EPR, and computational studies for all complexes. Further analyses indicate that both the sign and magnitude of zero-field splitting parameters experience a significant change (<i>D</i> ≥ ±150 cm^-1) by tuning of the axial and equatorial ligand field strength. Slow magnetic relaxation is observed for all compounds which is dominated by the Raman process involving both acoustic and optical phonons.