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 P1[combining macron]. 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 Ueff = 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.