Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12X11]- (X = F, Cl, Br, I, CN).

Electrophilic anions of type [B12X11]- posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study the reactivity of [B12X11]- with X = F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N2 as σ-donor-π-acceptors. Temperature-dependent formation of [B12X11NG]- indicate 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 [B12X11 ]- and steric effects. The binding of CO and N2 to [B12X11 ]- is significantly stronger. The B3LYP 0K attachment enthapies follow the order (X=F) > (X=CN) > (X=Cl) > (X=Br) > (X=I), in contrats to the NG series. The bonding motifs of [B12X11CO]- and [B12X11N2 ]- were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N2 stretching frequencies νCO and νN2 , respectively. Observed shifts of νCO and νN2 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 [B12X11]- reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo -borate anions.