Journal article

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

The first quant. evidence for the viability of three-dimensional arom. clusters involving d-orbitals in pseudo-octahedral coinage metal cages M6Lie2 (M = Cu, Tag, 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 analogs. The large neg. nucleus-independent chem. 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 MOs of the pseudo-octahedral clusters M6Li2 (M = Cu, Ag, Au) are analogous to those in similar octahedral clusters involving p-orbital delocalization (e.g., B6H62-). 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 anal. 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. [on SciFinder (R)]


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