八电子Pd4四面体团簇的电子结构稳定性分析

基于理论计算，我们报道了T_d对称性的[Pd₄(μ₃-SbH₃)₄(SbH₃)₄]团簇及一系列类似物的结构与成键。成键分析表明：每个Pd原子都是sp³杂化，其10个价电子与四个配体提供的8个价电子，满足18电子规则。并且，每个Pd原子与四个桥连的SbH₃配体可以形成四个离域的四中心两电子超级σ键或八中心两电子键。一方面，根据超原子网络模型，这个钯团簇可以描述成四个2电子的超原子网络。另一方面，凝胶模型表明，它可以合理化的作为电子组态是1S²1P⁶的8电子超原子。与此同时，d¹⁰…d¹⁰闭壳层相互作用在稳定Pd₄四面体结构中起到了关键性的作用。密度泛函理论计算表明：T_d对称性[Pd₄(μ₃-SbH₃)₄(SbH₃)₄]团簇表现出高度稳定性，具有充满的电子壳层，大的HOMO-LUMO带隙(2.84 eV)以及负的核独立化学位移(NICS)值。此外，基于[Pd₄(μ₃-SbH₃)₄(SbH₃)₄]结构与成键模式，我们设计了一系列稳定的类似物，其有可能被实验合成出来。

Electronic Stability of Eight-electron Tetrahedral Pd4 Clusters

Motivated by the unusual structure of the [Pd₄(μ₃-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 Pd₃ faces (J. Am. Chem. Soc. 2016, 138, 6964), we performed a computational study of the structure and bonding characteristics of the T_d [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] cluster and a series of its analogues. The T_d structure of the [Pd₄(μ₃-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 sp³ hybridized Pd atom contributed ten valence electrons, and eight valence electrons were provided by one terminal SbH₃ and three bridging μ₃-SbH₃ ligands. The [Pd₄(μ₃-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 σ bonds, four terminal Pd―Sb σ bonds, and four delocalized bonds. There were two bonding patterns of the eight delocalized electrons between the four capping Sb atoms and the Pd₄ 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^– [Pd₄(μ₃-SbH₃)₄] superatom with 1S²1P⁶ electronic configuration. The density functional theory (DFT) calculations showed that the T_d [Pd₄(μ₃-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 [Pd₄(μ₃-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 [Pd₄(μ₃-SbH₃)₄] motifs were much more negative than those of [Pd₄(SbH₃)₄] in the same range, revealing that the overall stability of [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] was likely derived from the local stability of Pd₄(μ₃-SbH₃)₄. Meanwhile, the d¹⁰…d¹⁰ interaction played a critical role in stabilizing the Pd₄ 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 [Pd₄(μ₃-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.