Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B12Cl11]−

It is common and chemically intuitive to assign cations electrophilic and anions nucleophilic reactivity, respectively. Herein, we demonstrate a striking violation of this concept: The anion [B12Cl11]− spontaneously binds to the noble gases (Ngs) xenon and krypton at room temperature in a reaction that is typical of “superelectrophilic” dications. [B12Cl11Ng]− adducts, with Ng binding energies of 80 to 100 kJ mol−1, contain B−Ng bonds with a substantial degree of covalent interaction. The electrophilic nature of the [B12Cl11]− anion is confirmed spectroscopically by the observation of a blue shift of the CO stretching mode in the IR spectrum of [B12Cl11CO]− and theoretically by investigation of its electronic structure. The orientation of the electric field at the reactive site of [B12Cl11]− results in an energy barrier for the approach of polar molecules and facilitates the formation of Ng adducts that are not detected with reactive cations such as [C6H5]+. This introduces the new chemical concept of “dipole-discriminating electrophilic anions.”