An optical basicity scale of various ionic liquids (IL) anions has been determined by UV-Vis absorption spectroscopy. The indicator complex [Ni(tmen)(acac)][B(Ph)4] (tmen = tetramethylethylendiamine, acac = acetylacetonate) shows to be a sensitive probe as the position of the 3A2g(F) → 3T1g(F) absorption maximum of square planar to octahedral Ni2+ shifts distinctly depending on the electron donor ability of the respective ionic liquid anion. If the Lewis basicity of the anion becomes stronger a change to a tetrahedral coordination around Ni2+ is observed. Here the 3T1(F) → 3T2(F) and 3T1(F) → 3T1(P) transitions are helpful for monitoring the IL anion basicity. Evaluating the optical spectra allows ordering the ionic liquids according to rising Lewis basicity: PF6− < BF4− < N(Tf)2− < OTf− ≪ HCOO− ≪ DCA− ≪ CF3COO− ≪ PO2(OEt)2− ≪ Cl−. No substantial influence of the ionic liquid counter cation on the Ni2+ absorption spectra could be detected. In consequence, [Ni(tmen)(acac)][B(Ph)4] allows evaluation of the donor power, hence, the Lewis basicity of the ionic liquid anion independent from the IL cation and an optical basicity scale of ionic liquid anions could be successfully established. In addition, [Ni(tmen)(acac)][B(Ph)4] turns out to be a sensitive probe to detect even small chloride impurities in ionic liquids which are a commonly encountered contamination in ILs.