From Very Strong to Inexistent Be-Be Bonds in the Interactions of Be₂ with π-Systems

Isolated Be₂ is a typical example of a weakly bound system, but interaction with other systems may give rise to surprising bonding features. The interactions between Be₂ and a set of selected neutral C_n H_n (n=2-8) π-systems have been analyzed through the use of G4 and G4MP2 ab initio methods, along with multireference CASPT2//CASPT2 calculations. Our results systematically show that the C_n H_n -Be₂ -C_n H_n clusters formed are always very stable. However, the nature of this interaction is completely different when the π-system involved is a closed shell species (n=2, 4, 6, 8), or a radical (n=3, 5, 7). In the first case, the interaction does not occur with the π-system as a whole, but with specific C centers yielding rather polar but strong C-Be bonds. Nonetheless, although the Be-Be distances in these complexes are similar to the ones in compounds with ultra-strong Be-Be bonds, a close examination of their electron density distribution reveals that no Be-Be bonds exist. The situation is totally different when the interaction involves two π-radicals, C_n H_n -Be₂ -C_n H_n (n=3, 5, 7). In these cases, a strong Be-Be bond is formed. Indeed, even though Be is electron deficient, the Be₂ moiety behaves as an efficient electron donor towards the two π-radicals, so that the different C_n H_n -Be₂ -C_n H_n (n=3, 5, 7) clusters are the result of the interaction between Be₂ ²⁺ and two L^- anions. The characteristics of these two scenarios do not change when dealing with bicyclic π-compounds, such as naphthalene and pentalene, because the interaction with the Be₂ moiety is localized on one of the unsaturated cycles, the other being almost a spectator.