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Motivated by the unusual structure of the [Pd-4(mu(3)-SbMe3)(4)(SbMe3)(4)] cluster, which is composed of a tetrahedral (T-d) Pd(0) core with four terminal SbMe3 ligands and four triply bridging SbMe3 ligands capping the four triangular ) Pd-3 faces (J. Am. Chem. Soc. 2016, 138, 6964), we performed a computational study of the structure and bonding characteristics of the T-d [Pd-4(mu(3)-SbH3)(4)(SbH3)(4)] cluster and a series of its analogues. The T-d structure of the [Pd-4(mu(3)-SbH3)(4)(SbH3)(4)] cluster could be explained by the cluster electron-counting rules based on the 18-electron rule for transition-metal centers; each sp(3) hybridized Pd atom contributed ten valence electrons, and eight valence electrons were provided by one terminal SbH3 and three bridging mu(3)-SbH3 ligands. The [Pd-4(mu(3)-SbH3)(4)(SbH3)(4)] cluster had a count of 104 valence electrons in total; chemical bonding analysis indicated that the cluster featured twenty electron lone pairs generated by d orbital of the four Pd atoms, twenty-four Sb-H sigma bonds, four terminal Pd-Sb sigma bonds, and four delocalized bonds. There were two bonding patterns of the eight delocalized electrons between the four capping Sb atoms and the Pd-4 core. The first pattern was based on the superatom-network (SAN) model, whereby the palladium cluster could be described as a network of four 2e(-) superatoms. The second pattern was based on the spherical jellium model, whereby the cluster could be rationalized as an 8e(-) [Pd-4(mu(3)-SbH3)(4)] superatom with 1S(2)1P(6) electronic configuration. The density functional theory (DFT) calculations showed that the T-d [Pd-4(mu(3)-SbH3)(4)(SbH3)(4)] cluster had a large HOMO-LUMO (HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital) energy gap (2.84 eV) and a negative nucleus-independent chemical shift (NICS) value (-12) at the center of the [Pd-4(mu(3)-SbH3)(4)(SbH3)(4)] cluster, indicating its high chemical stability and aromaticity. Furthermore, the NICS values in the range of 0-0.30 nm of the [Pd-4(mu(3)-SbH3)(4)] motifs were much more negative than those of [Pd-4(SbH3)(4)] in the same range, revealing that the overall stability of [Pd-4(mu(3)-SbH3)(4)(SbH3)(4)] was likely derived from the local stability of Pd-4(mu(3)-SbH3)(4). Meanwhile, the d(10)...d(10) interaction played a critical role in stabilizing the Pd-4 tetrahedron structure, which is similar to the aurophilicity in Au-Au clusters. It was also found that there is a large difference in the stability of transition metal and non-transition metal clusters with a tetrahedron structure. The structures and bonding patterns of the designed analogues were similar to those of [Pd-4(mu(3)-SbH3)(4)(SbH3)(4)]. To summarize, this study was relevant for deciphering the nature of the bonds in a tetrahedral complex with four cores and eight ligands, and predicting a series of analogues. It is expected that this work will provide more options for the synthesis of tetrahedral 4-core transition metal compounds.

Electronic Stability of Eight-electron Tetrahedral Pd<sub>4</sub> Clusters