On the nature of bonding in binary Be2O2 and Si2O2 clusters: rhombic four-center four-electron π and σ bonds

Abstract

The structural and electronic properties and chemical bonding of binary Be₂O₂ and Si₂O₂ clusters have been studied using quantum chemical calculations at the B3LYP level. For the Be₂O₂ 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 Be₂O₂ (¹A_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 Be₂O₂ and Si₂O₂ 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 Be₂O₂ and Si₂O₂ 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 Be₂O₂ and Si₂O₂ are considered to resemble the three-fold aromaticity in all-metal Al₄²⁻ 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 Be₂O₂ and Si₂O₂ clusters. We anticipate that the bonding concept should be applicable to additional molecular systems, including those with larger heterocyclic rings.