Three mononuclear octahedral Co(II) complexes are reported, [Co(py)(4)(SCN)(2)] (1), [Co(py)(4)(Cl)(2)]center dot 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, HFEPR, 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.