Octahedral and Tetrahedral Coinage Metal Clusters:  Is Three-Dimensional d-Orbital Aromaticity Viable?

• Published in: Inorganic chemistry 2006-01, Vol.45 (1), p.214-219
• Main Authors: Corminboeuf, Clémence, Wannere, Chaitanya S, Roy, Debjani, King, R. Bruce, Schleyer, Paul v. R
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic051576y

The first quantitative evidence for the viability of three-dimensional aromatic clusters involving d-orbitals in pseudo-octahedral coinage metal cages M6Li2 (M = Cu, Ag, Au) as well as in tetrahedral coinage metal cages M‘4Li4 (M‘ = Cu, Ag) was obtained computationally. These cages exhibit many features similar to those of their square planar M4Li2 analogues. The large negative nucleus-independent chemical shifts (NICS) at the cage centers indicate three-dimensional delocalization. This diatropic character arises mostly from d-orbital delocalization combined with substantial contributions from the lowest-valence orbitals. The bonding molecular orbitals of the pseudo-octahedral clusters M6Li2 (M = Cu, Ag, Au) are analogous to those in similar octahedral clusters involving p-orbital delocalization (e.g., B6H6 2-). The M‘4Li4 clusters exhibit two isomeric forms:  metal tetrahedral cages tetracapped by lithium cations on the outside [(M‘4)·4Li] and lithium tetrahedra on the inside capped by coinage metal atoms on each of the four faces [(Li4)·4M]. Whereas the (M‘4)·4Li type structure is preferred for copper, gold and silver favor the (Li4)·4M arrangement. NBO−NICS analysis shows that the large diatropic character in (M‘4)·4Li structures is due to the favorable contribution from both s- and d-orbitals, whereas the small NICS values in the center of (Li4)·4M are due only to the diatropic contributions from the s-orbitals.