Structure and bonding of the remarkable donor acceptor complexes XBeO X NH NMe CO N C H C H H H CO O 3 3 2 2 2 2 4 2 2 2

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 = NH 3 , NMe 3 , CO, N 2 , C 2 H 2 , C 2 H 4 , H 2 , H 2 CO and O 2 . The calculations show that BeO is a very strong Lewis acid. The X-BeO bond strength is between D e =69*5 kcal mo1 -1 for Me 3 NBeO and D 3 = 11*2 kcal mo1 -1 for π-bonded N 2 BeO. The calculated bond strength for the yet unknown donor-acceptor complex Me 3 NBeO is significantly higher than the strongest experimentally known main-group donor-acceptor complex Me 3 NA1C1 3 (D o = 47*5 kcal mo1 -1 ). Even the weak donor H 2 is bonded with D e = 18*5 kcal mo1 -1 . The compounds O 2 BeO and its isomer berylliumozonide BeO 3 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.