AbstractThe acidity‐enhancing effect of BH3 in gas‐phase phosphine⋅boranes compared to the corresponding free phosphines is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. Thus, the enhancement of the acidity of protic acids by Lewis acids usually observed in solution is also observed in the gas phase. For example, the gas‐phase acidities (GA) of MePH2 and MePH2⋅BH3 differ by about 118 kJ mol−1 (see picture).magnified imageThe gas‐phase acidity of a series of phosphines and their corresponding phosphine⋅borane derivatives was measured by FT‐ICR techniques. BH3 attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol−1). This acidity‐enhancing effect of BH3 is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High‐level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH3 association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH3 group is present. Our study also shows that deprotonation of ClCH2PH2 and ClCH2PH2⋅BH3 is followed by chloride departure. For the latter compound deprotonation at the BH3 group is found to be more favorable than PH2 deprotonation, and the subsequent loss of Cl− is kinetically favored with respect to loss of Cl− in a typical SN2 process. Hence, ClCH2PH2⋅BH3 is the only phosphine⋅borane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.