Dedicated to Prof. Andre Merbach on the occasion of his 65th birthdayThe ligand-exchange mechanism of solvated Be 2 cations has been studied by means of DFT calculations (RB3LYP/6-311 G**). Ligand exchange around [BeL 4 ] 2 , where L H 2 O, NH 3 , CO 2 , formaldehyde (H 2 CO), HCN, N 2 , and CO, was found to follow an associative interchange (I a ) process in all cases. The size of the activation barrier is almost independent of the type of donor atom, and depends mainly on the hybridization undergone by the donor atom. This, in turn, suggests that steric effects play a major role in solvent-and ligandexchange reactions in Be 2 systems.Introduction. ± Beryllium (Be), discovered in 1797 by Vauquelin and first isolated in 1828 by Wöhler and Bussy, plays a key role in a number of modern technologies: as a neutron moderator or reflector in nuclear reactors, in alloys with various metals in springs, electrical contacts, gyroscopes, satellites, and structural parts of the space shuttle, as metal foil in X-ray windows, or as oxide in ceramics [1 ± 3]. In astronomy, the traceability of Be allows the investigation of the evolution of stars [4], as well as its use as a cosmic chronometer [5]. However, applications of and research on Be and its compounds are severely hampered by the very high toxicities of these materials [6]. As a consequence, accurate data are relatively scarce.The reactivity of Be salts in solution is, in general, controlled by the lability of coordinated solvent molecules attached to Be 2 ions. This has been realized for a long time [7], and detailed studies on solvent-exchange reactions on Be 2 were performed by means of NMR techniques. Earlier mechanistic assignments were based on correlations between both stereochemistry and electron-donor ability of the solvent and its tendency to exchange with free solvent through a first-order and/or a secondorder pathway in non-coordinating polar diluents [8]. Available temperature-dependent data from which mechanistic conclusions were drawn on the basis of activation enthalpy and entropy were, at that point, still subject to criticism and not clear-cut to interpret. For this reason, Merbach and co-workers [9] have applied high-pressure NMR techniques to study solvent-exchange reactions on Be