Beryllium Bonding in the Light of Modern Quantum Chemical Topology Tools

We apply several modern quantum chemical topology (QCT) tools to explore the chemical bonding in well established beryllium bonds. By using the interacting quantum atoms (IQA) approach together with electron distribution functions (EDF) and the natural adaptive orbitals (NAdOs) picture, we show that, in agreement with orbital-based analyses, the interaction in simple σ and π complexes formed by BeX₂ (X = H, F, Cl) with water, ammonia, ethylene, and acetylene is dominated by electrostatic terms, albeit covalent contributions cannot be ignored. Our detailed analysis proves that several σ back-donation channels are relevant in these dimers, actually controlling the conformational preference in the π adducts. A number of one-electron beryllium bonds are also studied. Orbital invariant real space arguments clearly show that the role of covalency and charge transfer cannot be ignored.