Evidence for d Orbital Aromaticity in Square Planar Coinage Metal Clusters

Quantitative evidence for the existence of aromaticity involving the d orbitals of transition metals is provided for the first time. The doubly bridged square planar (D 4 h ) coinage metal clusters (M4Li2, M = Cu (1), Ag (2), and Au (3)) are characterized as aromatic by their substantial nucleus independent chemical shifts (NICS) values in the centers (−14.5, −14.1, and −18.6, respectively). Nevertheless, the participation of p orbitals in the bonding (and cyclic electron delocalization) of 1−3 is negligible. Instead, these clusters benefit strongly from the delocalization of d and to some extent s orbitals. The same conclusion applies to Tsipis and Tsipis' H-bridged D 4 h Cu4H4 ring (4). Canonical MO−NICS analysis of structures 1−3 shows the total diatropic d orbital contributions to the total NICS to be substantial, although the individual contributions of the five sets of filled d orbitals vary. The d orbital aromaticity of Cu4Li2 also is indicated by its atomization energy, 243.2 kcal/mol, which is larger than Boldyrev's doubly (σ and π) aromatic Al4Li2 (215.9 kcal/mol).