Molecules Meet Solids: From Wade–Mingos Clusters to Intermetalloid Clusters

Abstract

The common feature of molecular deltahedral borane clusters, molecules with a set of multiple bonds and metals is their electron deficiency, and they are all connected with the term “electron delocalization,” although with different meanings. The atomic and electronic structure of atom clusters is right between that of molecules and the extended bulk, and thus these clusters meet at the interface of molecules and solids.

In recent years intriguing examples of bare anionic homoatomic tetrel element clusters and more complex molecular metal clusters have been obtained from the reaction of tetrel element Zintl clusters in solution, and in the course of these studies a series of compounds has been established that describes the transition from molecules to intermetallic compounds. In this context the term intermetalloid clusters was coined. On the one hand, these clusters play a dominant role at the interface of molecular clusters and intermetallic solids, since they cover the entire area from rather simple borane-type species to intermetalloid clusters and intermetallic compounds, whose chemical bonding is so far hardly understood. On the other hand, there are extraordinary intermetallic compounds based on large intermetalloid clusters like [Sn@Cu₁₂@Sn₂₀]¹²⁻ that demand for an extension of the 8-N rule of Zintl phases which is derived from a superatom approach.

In this review, examples of borderline cases at the transition from “locally delocalized electrons” to delocalized electronic systems are summarized, with a special focus on examples that occur as anions in solution and in extended solids. The chosen cases allow a step-wise extension of the description of the chemical bond, starting from delocalized bonds in organic molecules, to delocalized bonds in deltahedral molecules, bare metal atom clusters, and rather complex intermetalloid clusters. The structural relationships between boranes B_nH_m^x− and tetrel element clusters [E_n]^x− are defined and applied to their derivatives, which include transition metal complexes. The similarities between the protonated species B₉H₁₂⁻ and [Si₉H₂]²⁻ and the bare tetrel clusters are emphasized. The introduction of ligand-stabilized transition metal fragments for cluster vertex expansion [E_nTL]^x− as well as the inclusion of metal atoms under formation of endohedral species [T@E_n]^x− is highlighted. The intriguing similarity of the local coordination environment of the transition metal atoms in [T@E_n]^x− clusters with n = 9 and 10 and in ternary Zintl phases and bare binary alloys with special emphasis on the oxidation state of the endohedral guest atom is discussed. The final part is dedicated to intermetalloid clusters with icosahedral structure. Using selected examples, possible reaction pathways to icosahedral building units are depicted. Specifically their relationship to compounds with highly coordinated atoms in intermetallic compounds is explored, and the formation of discrete and interpenetrating icosahedra is summarized. Even though the electronic structures of typical molecular units and larger intermetalloid clusters are different, the structures of boranes/carboranes like the tricommo{Ge₉Pd₃} and the tricommo-B₁₂ unit in elemental boron are related, and the unit [Pd₂@E₁₈]⁴⁻ (E = Ge, Sn) can be described as a “macropolyhedral” species. Finally, the transition from large intermetalloid clusters such as [Au₁₂Pb₄₄]⁸⁻ to Frank–Kasper phases is discussed.