On the nature of bonding in binary Be 2 O 2 and Si 2 O 2 clusters: rhombic four-center four-electron π and σ bonds

The structural and electronic properties and chemical bonding of binary Be 2 O 2 and Si 2 O 2 clusters have been studied using quantum chemical calculations at the B3LYP level. For the Be 2 O 2 cluster, the potential energy surface is probed by unbiased structural searches and the global-minimum structure was established using the B3LYP calculations, complemented by PBE0 and single-point CCSD(T) calculations for top isomers. The perfectly planar D 2h Be 2 O 2 ( 1 A g ) global minimum is well defined, being at least 3.64 eV lower in energy than alternative structures at the CCSD(T)//B3LYP/aug-cc-pVTZ level. Chemical bonding analyses show that D 2h Be 2 O 2 and Si 2 O 2 clusters possess the rhombic four-center four-electron (4c–4e) π bond, that is, the o-bond, a conception derived from electron-deficient boron oxide clusters lately. Furthermore, the Be 2 O 2 and Si 2 O 2 clusters also exhibit rhombic 4c–4e σ bonds, both for the radial and tangential σ frameworks (σ r and σ t ). The σ t framework is classified as an o-bond only formally, due to the secondary contribution from the Be/Si s component. The three-fold (π, σ r , and σ t ) o-bonds in Be 2 O 2 and Si 2 O 2 are considered to resemble the three-fold aromaticity in all-metal Al 4 2− dianions. A 4c–4e o-bond makes use of four O 2p electrons, which would otherwise be two lone-pairs, for a delocalized and completely bonding orbital, as well as a residual nonbonding orbital. Three-fold o-bonds thus greatly stabilize the binary Be 2 O 2 and Si 2 O 2 clusters. We anticipate that the bonding concept should be applicable to additional molecular systems, including those with larger heterocyclic rings.