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Superatoms


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the Wade‐Mingos rule, was developed by Wade [14, 15] and Mingos [16, 17] to account for the structure and bonding of polyhedral borane clusters (B n H n ). Because of electron deficiency, B cannot form conventional covalent bonds between adjacent pairs of atoms. Instead, it forms multicenter bonds with the valence electrons separated into external and skeletal electrons. The former forms traditional covalent bonds with ligands while the latter contributes to cage bonding. Consider a borane polyhedron, B n H n with n being the number of vertices. According to the Wade‐Mingos rule, 2n + 2, 2n + 4, and 2n + 8 electrons are needed to form closo, nido, and archano boranes, respectively. B12H12 2− is a well‐known example of a closo‐borane having an icosahedral geometry (Figure 2.22). Here, n = 12. Of the three valence electrons of B, one is involved in the covalent bonding with H while the other two are contributed to the cage bonding. Because 2n + 2 = 2 × 12 + 2 = 26 electrons are needed to stabilize the cage, B12H12 2− is stable as a dianion. Indeed, the electron affinities of the first and second electron are 4.56 and 0.9 eV, respectively.

Schematic illustration of geometry of B12H122-.

      Source: Adapted with permission from Jena and Sun [1]. © American Chemical Society.

Schematic illustration of pES spectrum of Al4H6 anion (left panel) and mass spectra of Al4Hm-. The inset shows the geometry of Al4H6-.

      Source: X. Li et al. [106]. © American Chemical Society.

      Pb12 2−, which is isoelectronic with Sn12 2−, was also found [58] to be a cage cluster whose stability can similarly be justified in terms of the Wade‐Mingos rule. Named “plumbaspherene,” Pb12 2− cage has a diameter of 6.29 Å and can accommodate a metal atom inside it. This is consistent with an earlier experiment where an extremely stable AlPb12 + cluster was synthesized in the gas phase. AlPb12 + can be viewed as Al3 +: Pb12 2− just as KSn12 was viewed as K+: Sn12 2−. Note that a series of endohedral cage compounds, M@Pb12 2− (M = Ni, Pd, Pt), stabilized by K+ counterions, have been synthesized in solution and in crystalline form. Both of their icosahedral symmetries have been confirmed [85, 86] by X‐ray diffraction and nuclear magnetic resonance (NMR) experiments.

Schematic illustration of optimized structure of (a) Sn12-, (b) Sn122-, and (c) KSn12-.

      Source: Cui et al. [59]. © American Chemical Society.