[M₁₆Ni₂₄(CO)₄₀]^4–: Coinage Metal Tetrahedral Superatoms as Useful Building Blocks Related to Pyramidal Au₂₀ Clusters (M = Cu, Ag, Au). Electronic and Bonding Properties from Relativistic DFT Calculations

Characterization of the tetrahedral Au₂₀ structure in the gas phase remains a major landmark in gold cluster chemistry, where further efforts to stabilize this bare 20-electron superatom in solution to extend and understand its chemistry have failed so far. Here, we account for the structural, electronic, and bonding properties of [M₁₆Ni₂₄(CO)₄₀]^4– (M = Cu, Ag, Au) observed in solution for gold and silver. Our results show a direct electronic relationship with Au₂₀, owing that such species share a common tetrahedral [M₁₆]^4– central core with a 1S²1P⁶1D¹⁰2S² jellium configuration. In the case of Au₂₀, the [Au₁₆]^4– core is capped by four Au⁺ ions, whereas in [M₁₆Ni₂₄(CO)₄₀]^4– it is capped by four Ni₆(CO)₁₀ units. In both cases, the capping entities are a full part of the superatom entity, where it appears that the free (uncapped) [M₁₆]^4– species must be capped for further stabilization. It follows that the Ni₆(CO)₁₀ units in [M₁₆Ni₂₄(CO)₄₀]^4– should not be considered as external ligands as their bonding with the [M₁₆]^4– core is mainly associated with a delocalization of the 20 jellium electrons onto the Ni atoms. Thus, the [M₁₆Ni₂₄(CO)₄₀]^4– species can be seen as the solution version of tetrahedral M₂₀ clusters, encouraging experimental efforts to further develop the chemistry of such complexes as M(111) finite surface section structures, with M = Ag and Au and, particularly promising, with M = Cu. Furthermore, optical properties were simulated to assist future experimental characterization.