The frequency dependence of the in-phase (χ′) and the out-of-phase (χ″) magnetic susceptibility for complexes [Co(1-3)](ClO4)2 was measured in the temperature range 1.8 -10 K under an oscillating (1-1500 Hz) field of 1.55 Oe (0.155 mT). In the absence of an external static field, none of the complexes displayed maxima in the out-of-phase susceptibility. Application of a static field reduces the quantum tunneling of magnetization (QTM) effect by removing the degeneracy between microstates and thus out-of-phase maxima for all complexes are observed (Figures ). These shift to low-frequency with the increase in the magnetic field strength reaching a maximum shift under an applied field of 100-250 mT, considered to be the optimal field under which to observe the slow magnetic relaxation. Strongly temperature and frequency dependent ac susceptibility signals characteristic of single ion magnet (SIM) behavior were observed in all cases (Figure ). This characteristic field-induced SIM behavior, displayed by all complexes, is exemplified for [Co( )](ClO4)2 in Figure .