Syntheses of a novel pseudotetrahedral four-coordinate mononuclear Co(II) complex that has two halved phthalocyanine moieties as the ligands, [Co(half-Pc)(2)] (1), and its magnetic properties as a single molecule magnet (SMM) are reported. A one pot reaction of phthalonitrile and lithium methoxide followed by the coordination to a Co(II) ion gave 1 as an orange solid in a moderate yield. X-ray crystallography on 1 reveals tetragonally distorted coordination geometry around the Co(II) ion. The M-HT-1 plots suggest that 1 has large axial magnetic anisotropy. The ac magnetic susceptibility data of the magnetically diluted 1 (dill) clearly show that the complex acts as an SMM even in the absence of the external static magnetic field (H-dc). The influence of intermolecular and intramolecular interactions for the magnetic relaxation behaviors has been discussed by comparison of the magnetic data of 1 and dil. 1. The Orbach process is suggested as the predominant mechanism of magnetic relaxations in the high-temperature range, and the Arrhenius plots provide the effective relaxation energy barrier and pre-exponential factor of U-eff = 54.0 cm(-1) and tau(0) = 3.17 X 10(-10) s, respectively. The direct estimation of the axial anisotropic parameter of 1 was successfully performed by high-field, multifrequency ESR measurements up to 55 T and 2.5 THz. The evaluated axial zero-field splitting (ZFS) energy of 57.0 cm(-1) is comparable to the U-eff energy, confirming that the magnetic relaxations are initiated by the thermal excitation from the ground vertical bar M-S > = vertical bar +/- 3/2) states to the vertical bar +/- 1/2 > states in the high-temperature range. The results of the ab initio calculations based on the CAS(7,5) SCF wave functions indicate that the ground states of 1 consist mainly of vertical bar M-S > = vertical bar +/- 3/2 > states, while the first excited states are the mixture of vertical bar M-S > = vertical bar+1/2 > and vertical bar-1/2 >.