Electronic Stability of Phosphine-Protected Au20 Nanocluster: Superatomic Bonding

A recent experiment reported that a newly crystallized phoSphine-Protected Au-20 nanocluster [Au-20(PPhy(2))(10)Cl-4]Cl-2 [PPhpy2 = bis(2-pyridyl)phenylphosphine] owns a very, stable Au-20 tore, but the number of valence electrons of the Au-20 core is 14e, which is not predicted by the superatom model. So we apply the density functional theory to further study this cluster from its molecular orbital and chemical bonding. The results suggest that the Au-20((+6)) core is an analogue of the F-2 molecule based on the super valence bond model, and the 20-center-14-electron Au-20((+6)) core can be taken as a superatomic molecule bonded by two 11-center-7-electron superatoms, where the two 11c superatoms share two Au atoms and two electrons to meet an 8-electron closed shell for each. The electronic shell closure enhances the stability of the Alin core, besides the PN bridges. Exceptionally, the theoretical HOMO-LUMO gap (1.03 eV) disagrees with the experimental value (2.24 eV), and some possible reasons for this big difference are analyzed in this paper.