Aromaticity of Some Metal Clusters: A Different View from Magnetic Ring Current.

The aromatic character of some small planar metallic clusters was revisited with an emphasis on their σ electrons. In contrast to previous reports, our approach based on magnetic ring current as an indicator for aromaticity points out that the σ electron delocalization in molecules behaves as an important contributor to their thermodynamic stability. Ring current maps were constructed using electron densities obtained from density functional theory calculations with the B3LYP functional and the 6-311G(d) basis set. Diatropic currents were further confirmed by an analysis of the symmetry of electronic excitations involved. The triatomic B cycle is found to maintain a double σ and π aromaticity when it forms the [B(NN)] and [B(CO)] complexes. The planar pentacoordinated carbon clusters including C@Al, C@AlBe, C@BeH, C@BeLi, and C@BeHLi are σ aromatic rather than π aromatic as previously assigned. The mixed copper clusters CuSi and CuGe are found to be σ aromatic compounds. The copper hydrides CuH can better be regarded as nonaromatic rather than aromatic compounds. The ring current indicator reveals the σ aromatic feature for Be@BeH and Be@BeH clusters, whereas this criterion shows a double aromaticity of Be@B and Be@B. Overall, the present study points out again the importance of σ electrons in determining the bonding characteristics of metallic clusters, and they should equally be considered as a key element.