AbstractSuperelectrophilic anions constitute a special class of molecular anions that show strong binding of weak nucleophiles despite their negative charge. In this study, the binding characteristics of smaller gaseous electrophilic anions of the types [B₆X₅]⁻ and [B₁₀X₉]⁻ (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B₁₂X₁₁]⁻. The positive charge of vacant boron increases from [B₆X₅]⁻ via [B₁₀X₉]⁻ to [B₁₂X₁₁]⁻, as evidenced by increasing attachment enthalpies towards typical σ‐donor molecules (noble gases, H₂O). However, this behavior is reversed for σ‐donor–π‐acceptor molecules. [B₆Cl₅]⁻ binds most strongly to N₂ and CO, even more strongly than to H₂O. Energy decomposition analysis confirms that the orbital interaction is responsible for this opposite trend. The extended transition state natural orbitals for chemical valence method shows that the π‐backdonation order is [B₆X₅]⁻>[B₁₀X₉]⁻>[B₁₂X₁₁]⁻. This predicted order explains the experimentally observed red shifts of the CO and N₂ stretching fundamentals compared to those of the unbound molecules, as measured by infrared photodissociation spectroscopy. The strongest red shift is observed for [B₆Cl₅N₂]⁻: 222 cm⁻¹. Therefore, strong activation of unreactive σ‐donor–π‐acceptor molecules (commonly observed for cationic transition metal complexes) is achieved with metal‐free molecular anions.