The properties of lanthanide single-ion magnets (SIMs) is delicately balanced to the crystal field (CF) of the lanthanide ion in (typically) low symmetry surroundings. We have synthesized a series of high-symmetry Er(III) SIMs processing long spin-relaxation times. In order to understand the CF perturbations imposed by chemical functionalization of the surrounding ligand two extreme cases are under study. The study of these two compounds sets the outer limits for the tuning of the CF potential.