Relevance of π‐Backbonding for the Reactivity of Electrophilic Anions [B(12)X(11)](−) (X=F, Cl, Br, I, CN)

Electrophilic anions of type [B(12)X(11)](−) posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B(12)X(11)](−) with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ‐donors and (ii) CO/N(2) as σ‐donor‐π‐acceptors. Temperature‐dependent formation of [B(12)X(11)NG](−) indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B(12)X(11)](−) and steric effects. The binding of CO and N(2) to [B(12)X(11)](−) is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B(12)X(11)CO](−) and [B(12)X(11)N(2)](−) were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N(2) stretching frequencies [Formula: see text] and [Formula: see text] , respectively. Observed shifts of [Formula: see text] and [Formula: see text] are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π‐backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B(12)X(11)](−) reactivety dependent on the substituent X and provides first systematic data on π‐backdonation from delocalized σ‐electron systems of closo‐borate anions.