Compounds 2 and 3 were, however, found to be inert to various fluorination protocols, including the use of KF/18-crown-6, trimethyltin fluoride (Me3SnF) and tetramethylammonium fluoride (TMAF), each of which has been previously shown to offer the possibility for introducing fluoride into the coordination sphere of calcium. Presumably, the net anionic charge borne by the (anionic) calcium-containing component of 2 renders the corresponding dianionic Ca-F-containing species inaccessible. Neutral complex 3, on the other hand, yields HF and proteo-ligand under similar conditions (see Figures and in SI for details).