δ Aromaticity in [Ta3O3]−

The concept of aromaticity was introduced into organic chemistry to describe delocalized p bonding in planar, cyclic, and conjugate molecules possessing (4n+2) p electrons. In recent years, this concept has been advanced into main-group molecules including organometallic compounds with cyclic cores of metal atoms and, in particular, all-metal clusters. It has been shown that main-group clusters may exhibit multiple aromaticity (s and p), multiple antiaromaticity (s and p), and conflicting aromaticity (s aromaticity and p antiaromaticity or s antiaromaticity and p aromaticity). Here, we report experimental and theoretical evidence of d aromaticity, which is only possible in transition-metal systems. It is discovered in the [Ta3O3] cluster through combined photoelectron spectroscopy and ab initio studies. Well-resolved low-lying electronic transitions are observed in the photoelectron spectra of [Ta3O3] and are compared with ab initio calculations, which show that the [Ta3O3] cluster has a planar D3h triangular structure. Chemical-bonding analyses reveal that among the five valence molecular orbitals involved in the multicenter metal–metal bonding, there is a completely bonding d and p orbital formed from the 5d atomic orbitals of Ta. The totally delocalized multicenter d bond renders d aromaticity for [Ta3O3] and represents a new mode of chemical bonding. [Ta3O3] is the first d-aromatic molecule confirmed experimentally and theoretically, which suggests that d aromaticity may exist in many multinuclear, lowoxidation-state transition-metal compounds. In 1964, Cotton and co-workers published a milestone work on K2[Re2Cl8]·2H2O, [7] in which they showed the presence of a new type of chemical bond—a d bond between the two Re atoms. Since then, a branch of inorganic chemistry has been developed that involves multiple metal–metal bonding with bond orders higher than three, the maximum allowed for main-group systems. Power and co-workers recently reported the synthesis of a Cr2 compound with a quintuple bond (spd) between the two Cr atoms. This work, along with recent quantum chemical studies of multiple bonds in U2 and [Re2Cl8] 2 , has generated renewed interest in multiple metal–metal bonding. The presence of d bonds between two transition-metal atoms suggests that multicenter transition-metal species with a completely delocalized cyclic d bond may exist, thus raising the possibility of d aromaticity analogous to p or s aromaticity in main-group systems. We have been interested in understanding the electronic structure and chemical bonding of early transition-metal oxide clusters as a function of size and composition, and in using them as potential molecular models for oxide catalysts. During our investigation of tantalum oxide clusters, we found the presence of d aromaticity in the [Ta3O3] cluster, in which each Ta atom is in a low oxidation state of Ta and still possesses three electrons for Ta–Ta bonding. The experiment was conducted by using a magneticbottle-type photoelectron spectroscopy apparatus equipped with a laser vaporization cluster source. [TamOn] clusters with various compositions were produced by laser vaporization of a pure tantalum disk target in the presence of a helium carrier gas seeded with O2, and were size-separated by time-of-flight mass spectrometry. The [Ta3O3] species was mass-selected and decelerated before photodetachment by a pulsed laser beam. Photoelectron spectra were obtained at two relatively high photon energies, 193 nm (6.424 eV) and 157 nm (7.866 eV), to guarantee access to all valence electronic transitions (Figure 1). Three well-resolved bands (X, A, and B) were observed at the lower-binding-energy side. The X band is much more intense and shows a discernible splitting at 193 nm (Figure 1a). Surprisingly, no well-defined electronic transitions were observed beyond 3.7 eV, where continuous signals were present, probably as a result of multielectron transitions. The vertical detachment energies (VDEs) of the observed transitions at the low-bindingenergy side are given in Table 1, where they are compared with theoretical calculations by two different methods. [*] Dr. H. J. Zhai, Prof. Dr. L. S. Wang Department of Physics Washington State University 2710 University Drive, Richland, WA 99354 (USA) and Chemical & Materials Sciences Division Pacific Northwest National Laboratory MS K8–88, P.O. Box 999, Richland, WA 99352 (USA) Fax: (+1)509-376-6066 E-mail: ls.wang@pnl.gov