title: Evidence for d orbital aromaticity in square planar coinage metal clusters.
abstract: 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 (M(4)Li(2), 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)() Cu(4)H(4) 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 Cu(4)Li(2) also is indicated by its atomization energy, 243.2 kcal/mol, which is larger than Boldyrev's doubly (sigma and pi) aromatic Al(4)Li(2) (215.9 kcal/mol).
authors.affiliation: Center for Computational Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia 30602, USA.
authors.forename: Chaitanya S
authors.initials: CS
authors.lastname: Wannere
authors.forename: Clémence
authors.initials: C
authors.lastname: Corminboeuf
authors.forename: Zhi-Xiang
authors.initials: ZX
authors.lastname: Wang
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Wodrich
authors.forename: R Bruce
authors.initials: RB
authors.lastname: King
authors.forename: Paul V R
authors.initials: PV
authors.lastname: Schleyer