[M16Ni24(CO)(40)](4-): Coinage Metal Tetrahedral Superatoms as Useful Building Blocks Related to Pyramidal Au-20 Clusters (M = Cu, Ag, Au). Electronic and Bonding Properties from Relativistic DFT Calculations

Characterization of the tetrahedral Au-20 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 [M-16](4-) central core with a 1S(21)P(61)D(10)2S(2) jellium configuration. In the case of Au-20, the [Au-16](4-) core is capped by four Au+ ions, whereas in [M(16N)i(24)(CO)(40)](4-) it is capped by four Ni-6(CO)(10) units. In both cases, the capping entities are a full part of the superatom entity, where it appears that the free (uncapped) [M-16](4-) species must be capped for further stabilization. It follows that the Ni-6(CO)(10) units in [M16Ni24(CO)(40)](4-) should not be considered as external ligands as their bonding with the [M-16](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 M-20 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.