Surprising Electronic Structure of the BeH– Dimer: a Full-Configuration-Interaction Study

The electronic structure of the beryllium hydride anion, BeH–, was investigated at valence full-configuration-interaction (FCI) level, using large cc-pV6Z basis sets. It appears that there is a deep change of the wave function nature as a function of the internuclear distance: the ion structure goes from a weakly bonded Be···H– complex, at long distance, to a rather strongly bonded system (more than 2 eV) at short distance, having a (:Be–H)− Lewis structure. In this case, it is the beryllium atom that formally bears the negative charge, a surprising result in view of the fact that it is the hydrogen atom that has a larger electronegativity. Even more surprisingly, at very short distances the average position of the total electronic charge is close to the beryllium atom but on the opposite side with respect to the hydrogen position.