How many electrons to stabilize the icosahedral Cu₅₅ core in ligated nanoclusters? The example of [Cu₅₅(NHC)₆]

While ligand-protected clusters with M₅₅ icosahedral cores are rarely encountered so far, DFT calculations have been performed on a series of bare and ligated Cu₅₅ clusters, including the structurally characterized [Cu₅₅(IDipp)₆]. Calculations indicate that the best closed-shell superatomic electron counts for such species are 48, 50, 52 and 56. None of them is a superatom "magic" number. The closest "magic" number, namely 58, which would correspond to full occupation of the 1G level, is highly disfavored. The 48, 50, 52 and 56 counts correspond to 1G partial occupations and can be rationalized from Jahn-Teller distortions away from R³ or I_h symmetry. Similar calculations on related Au₅₅ clusters provided comparable results, with the exception of the 52-electron count, which is not favored as a closed-shell with gold. Neutral ligands such as NHCs, are expected to stabilize efficiently the closed-shell 56-electron count, providing they are able to sterically screen all the "exposed" metal atoms.