A rational approach of modulating the easy-plane magnetic anisotropy of mononuclear pentagonal bipyramidal Coll single molecule magnets (SMMs) has been revealed in this paper. A class of three new pentagonal-bipyramidal complexes with formulas [Co(H(2)claps)(MeOH)(2)] (1), [Co(H(4)daps)(NCS)(MeOH)]-(ClO4)-(MeOH) (2), and [Co(H(4)daps)(NCS)(2)] center dot (MeOH)(2) (3) (H(4)claps = 2,6-bis(1-salicyloylhydrazonoethyl) pyridine) were studied. In these complexes, the axial positions are successively replaced by different 0 and N donar ligands in a systematic way. Detailed magnetic measurements disclose the existence of large easy-plane magnetic anisotropy and field-induced slow magnetic relaxation behavior. Both experimental and ab initio theoretical calculations display that easy-plane magnetic anisotropy is maintained upon variation of coordination environments. Nevertheless, the magnitude of the D value was found to be increased in the case of weaker axially coordinated 6 donor ligands and a more symmetrical equatorial ligand. Additionally, the detailed investigation of field and temperature dependence of relaxation time revealed that quantum tunnelling of magnetization is the predominant process for slow magnetic relaxation and the Raman process is significant which explicates the thermal dependence. Magnetic dilution experiments have been performed to eliminate the possible influence of intermolecular interactions on magnetic behaviors of adjacent Coll centers.