This paper describes a tetrahedral mononuclear Co(II) complex [CoL₂](ClO₄)₂ (1) in which L = 2,9-diphenyl-1,10-phenanthroline. The structure of 1, which was determined by single crystal X-ray diffraction, indicates that it exists in the triclinic space group P. Magnetic property studies were conducted by reduced magnetization measurements, ab initio calculations and X-band EPR experiments, the results of which revealed a large zero-field splitting, with D ∼ −45.9 cm⁻¹. The Arrhenius equation indicates that the kinetic energy barrier of 1 is U_eff = 46.9 cm⁻¹. This study describes a very rare case of a Co(II) single ion magnet (SIM) that is purely tetrahedrally coordinated by pyridine like ligands.