Chemical Bonding and σ-Aromaticity in Charged Molecular Alloys: [Pd(2)As(14)](4−) and [Au(2)Sb(14)](4−) Clusters

We report a computational study on the structures and bonding of a charged molecular alloy D (2h) [Pd(2)As(14)](4−) (1), as well as a model D (2h) [Au(2)Sb(14)](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 X(7) (X = As, Sb) cages, which are interconnected via a M(2) (M = Pd, Au) dumbbell, featuring two distorted square-planar MX(4) 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 AuSb(4) fragment). Thus, 1 and 2 conform to the (4n + 2) Hückel rule, for n = 0 and 1, respectively, rendering them σ-aromaticity.