Structure and bonding of the remarkable donor-acceptor complexes XBeO (X = NH₃, NMe₃, CO, N₂, C₂H₂, C₂H₄, H₂, H₂CO, O₂)

Quantum mechanical calculations at the MP4/6-311G(2df,2pd)//MP2/6-31G(d,p) level of theory are reported for the compounds XBeO with X = NH₃, NMe₃, CO, N₂, C₂H₂, C₂H₄, H₂, H₂CO and O₂. The calculations show that BeO is a very strong Lewis acid. The X - BeO bond strength is between D_e = 69:5 kcal mol^-1 for Me₃NBeO and D_e = 11:2 kcal mol^-1 for π-bonded N₂BeO. The calculated bond strength for the yet unknown donor-acceptor complex Me₃NBeO is significantly higher than the strongest experimentally known main-group donor-acceptor complex Me₃NAlCl₃(D_o = 47:5 kcal mol^-1). Even the weak donor H₂ is bonded with D_e = 18:5 kcal mol^-1. The compounds O₂BeO and its isomer berylliumozonide BeO₃ should not be considered as donor-acceptor complexes. The results of the CDA method show that the donor-acceptor interactions in terms of orbital mixing are mainly described by the mixing of occupied orbitals of X with vacant orbitals of BeO, while the mixing of occupied orbitals of BeO with vacant orbitals of X is negligible. The topological analysis of the electronic charge distribution and the NBO partitioning scheme demonstrate that the X-BeO bonds have little or no covalent character; the bonds are caused by electrostatic attraction. The charge concentration at the donor atoms in the stronger bonded compounds is significantly deformed towards the beryllium atom.