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] (1), [Co(py)4(Cl)2]·H2O (2), and [Co(py)4(Br)2] (3), that exhibit different distortions with compression (1) or elongation (2 and 3) 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 (D ≥ ±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.