Pentacoordinate complex [(CoLCl2)-Cl-3] with a tridentate antenna-like ligand L-3 forms a dimer held by short pi-pi stacking with head-to-head contacts at 3.4 angstrom. The direct-current (dc) magnetic susceptibility and magnetization data confirm weak ferromagnetic interaction and a large-magnetic anisotropy, D/hc = 150 cm(-1) and E/hc = 11.6 cm(-1). The system shows superparamagnetic behavior at low temperature that depends upon the applied magnetic field. At B-dc = 0.2 T, a low-frequency peak at the out-of-phase susceptibility is seen (v similar to 0.3 Hz), whereas the onset of the second peak appears at v > 1500 Hz, indicating the existence of two slow relaxation processes.