We report on the synthesis, structure and investigation of magnetic properties of a series of six heteroleptic complexes of the general formula [Ln(Tpm)X3]·yMeCN (Tpm = tris(3,5-dimethylpyrazolyl)methane; Ln = Tb, Dy, Er; X- = NO3-, Cl- and y = 1 or 2). We demonstrate that the observation of a field-induced slow relaxation of magnetization is highly dependent on the anion's nature. While nitrate moieties appear to be suitable to stabilize the oblate electronic density of Dy3+, chloride ions generate an equatorial crystal-field allowing the slow relaxation of the prolate Er3+ ions.