To test the coordination symmetry effect on the magnetization-reversal barrier trend of Er-III-based single-ion Magnets, the C-2-symmetric organolanthanide anion [Er(Cp)(2)((OBu)-Bu-t)(2)](-) has been incorporated with different countercations, resulting in two structures, namely, the discrete [K-2(Cp)(18-C-6)(2)][Er(Cp)(2)((OBu)-Bu-t)(2)] (1) and the polynieric [ErK2(Cp)-3((OBu)-Bu-t)(2)(THP)(2)](n) (2), where 18-C-6 = 18-crown-6 ether and Cp = cyclopentadienide. Surprisingly, the polymeric 2 exhibits much stronger field,induced Magnetization relaxing behavior compared to the monomeric 1. Such disparate dynamic Magnetism is attributable to the subtle coordination environmental perturbations of the central Er-III ions.