Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B 12 Cl 11 ] −

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 [B 12 Cl 11 ] − spontaneously binds to the noble gases (Ngs) xenon and krypton at room temperature in a reaction that is typical of “superelectrophilic” dications. [B 12 Cl 11 Ng] − 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 [B 12 Cl 11 ] − anion is confirmed spectroscopically by the observation of a blue shift of the CO stretching mode in the IR spectrum of [B 12 Cl 11 CO] − and theoretically by investigation of its electronic structure. The orientation of the electric field at the reactive site of [B 12 Cl 11 ] − 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 [C 6 H 5 ] + . This introduces the new chemical concept of “dipole‐discriminating electrophilic anions.”