Nb₂©Au₆: a molecular wheel with a short NbNb triple bond coordinated by an Au₆ ring and reinforced by σ aromaticity

Abstract

We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic Nb₂Au₆ ^- cluster. Theoretical calculations, in conjunction with the experimental data, reveal that Nb₂Au₆ ^-/0 possess high-symmetry <i>D</i> _6h structures featuring a Nb-Nb axis coordinated equatorially by an Au₆ ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb₂ moiety in Nb₂©Au₆, as well as five totally delocalized σ bonds. The Nb[triple bond, length as m-dash]Nb triple bond is strengthened significantly by the delocalized σ bonds, resulting in an extremely short Nb-Nb bond length comparable to the quintuple bond in gaseous Nb₂. The totally delocalized σ bonding in Nb₂©Au₆ is reminiscent of σ aromaticity, representing a new bonding mode in metal-ligand systems. The unusually short Nb-Nb bond length in Nb₂©Au₆ shows that the Au₆ ring can serve as a bridging ligand to facilitate multiple bonding in transition metal dimers <i>via</i> delocalized σ bonding.