Chemical Bonding and σ-Aromaticity in Charged Molecular Alloys: [Pd2As14]4- and [Au2Sb14]4- Clusters.

We report a computational study on the structures and bonding of a charged molecular alloy D 2h [Pd2As14]4− (1), as well as a model D 2h [Au2Sb14]4− (2) cluster. Our effort makes use of an array of quantum chemistry tools: canonical molecular orbital analysis, adaptive natural density partitioning, natural bond orbital analysis, orbital composition analysis, and nucleus independent chemical shift calculations. Both clusters consist of two X7 (X = As, Sb) cages, which are interconnected via a M2 (M = Pd, Au) dumbbell, featuring two distorted square-planar MX4 units. Excluding the Pd/As or Au/Sb lone-pairs, clusters 1 and 2 are 50- and 44-electron systems, respectively, of which 32 electrons are for two-center two-electron (2c-2e) As-As or Sb-Sb σ bonds and an additional 16 electrons in 1 for 2c-2e Pd-As σ bonds. No covalent Pd-Pd or Au-Au bond is present in the systems. Cluster 1 is shown to possess two globally delocalized σ electrons, whereas 2 has two σ sextets (each associated with an AuSb4 fragment). Thus, 1 and 2 conform to the (4n + 2) Huckel rule, for n = 0 and 1, respectively, rendering them σ-aromaticity.