title: On the nature of chemical bonding in the all-metal aromatic [Sb3Au3Sb3](3-) sandwich complex.
abstract: In a recent communication, an all-metal aromatic sandwich [Sb3Au3Sb3](3-) was synthesized and characterized. We report herein a density-functional theory (DFT) study on the chemical bonding of this unique cluster, which makes use of a number of computational tools, including the canonical molecular orbital (CMO), adaptive natural density partitioning (AdNDP), Wiberg bond index, and orbital composition analyses. The 24-electron, triangular prismatic sandwich is intrinsically electron-deficient, being held together via six Sb-Sb, three Au-Au, and six Sb-Au links. A standard, qualitative bonding analysis suggests that all CMOs are primarily located on the three Sb3/Au3/Sb3 layers, three Au 6s based CMOs are fully occupied, and the three extra charges are equally shared by the two cyclo-Sb3 ligands. This bonding picture is referred to as the zeroth order model, in which the cluster can be formally formulated as [Sb3(1.5+)Au3(3-)Sb3(1.5+)](3-) or [Sb3(0)Au3(3-)Sb3(0)]. However, the system is far more complex and covalent than the above picture. Seventeen CMOs out of 33 in total involve remarkable Sb → Au electron donation and Sb ← Au back-donation, which are characteristic of covalent bonding and effectively redistribute electrons from the Sb3 and Au3 layers to the interlayer edges. This effect collectively leads to three Sb-Au-Sb three-center two-electron (3c-2e) σ bonds as revealed in the AdNDP analyses, despite the fact that not a single such bond can be identified from the CMOs. Orbital composition analyses for the 17 CMOs allow a quantitative understanding of how electron donation and back-donation redistribute the charges within the system from the formal Sb3(0)/Au3(3-) charge states in the zeroth order model to the effective Sb3(1.5-)/Au3(0) charge states, the latter being revealed from the natural bond orbital analysis.
authors.affiliation: Nanocluster Laboratory, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China. hj.zhai@sxu.edu.cn.
authors.forename: Xue-Rui
authors.initials: XR
authors.lastname: You
authors.forename: Wen-Juan
authors.initials: WJ
authors.lastname: Tian
authors.forename: Da-Zhi
authors.initials: DZ
authors.lastname: Li
authors.forename: Ying-Jin
authors.initials: YJ
authors.lastname: Wang
authors.forename: Rui
authors.initials: R
authors.lastname: Li
authors.forename: Lin-Yan
authors.initials: LY
authors.lastname: Feng
authors.forename: Hua-Jin
authors.initials: HJ
authors.lastname: Zhai