In this work, the effects of ligand field strength as well as the metal coordination geometry on magnetic anisotropy of pentacoordinated Co-II complexes have been investigated using a combined experimental and theoretical approach. For that, a strategic design and synthesis of three pentacoordinate Co-II complexes [Co(bbp)Cl-2]center dot(MeOH) (1), [Co(bbp)Br-2]center dot(MeOH) (2), and [Co(bbp)(NCS)(2)] (3) has been achieved by using the tridentate coordination environment of the ligand in conjunction with the accommodating terminal ligands (i.e., chloride, bromide, and thiocyanate). Detailed magnetic studies disclose the occurrence of slow magnetic relaxation behavior of Co-II centers with an easy-plane magnetic anisotropy. A quantitative estimation of ZFS parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the alpha -> beta (SOMO-SOMO) excitation contributes almost entirely to the total ZFS values for all complexes. It is worth noting that the excitation pertaining to the most positive contribution to the ZFS parameter is the d(xy) -> d(x2-y2) excitation for complexes 1 and 2, whereas for complex 3 it is the d(z2) -> d(x2-y2) excitation.