Charge transfer in beryllium bonds and cooperativity of beryllium and halogen bonds. A new perspective

The main characteristics of beryllium bonds formed by the interaction of different Lewis bases with BeX2 (X = H, F) moieties have been analyzed by means of the Charge Displacement (CD) function. This analysis is systematically compared with that provided by other approaches based on the topology of the electron density, namely the quantum theory of atoms in molecules (QTAIM) and the electron localization function (ELF). The CD scheme provides a quantitative description of the charge transfer that gives rise to the formation of beryllium bonds. For systems of suitable symmetry, its decomposition into symmetry contributions allows to easily identify the mechanisms involved in the charge transfer process, as well as to quantify possible back-donations. The CD function analysis also provides a clear quantitative description of cooperativity between the beryllium and halogen bonds in ternary F2Be:FCl:N-base (N-base = NH3, NHCH2, NCH) complexes, confirming the trends obtained by the QTAIM and ELF methods. The different viewpoints each of these methodologies provide are clearly complementary, the CD being the only one that permits to quantify the charge transfer from the Lewis base to the Lewis acid.