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43101-AC6
Probing Aromaticity in Transition Metal Clusters

Alexander I. Boldyrev, Utah State University

           During the second year of our grant we concentrated on the search for systems with novel type of aromaticity - d-aromaticity. We previously showed that aromaticity/antiaromaticity in metal systems has very specific flavors if compared with organic compounds.  The striking feature of chemical bonding in metal systems is the possibility of the multi-fold nature of aromaticity, antiaromaticity, and conflicting aromaticity. When only s-atomic orbitals (AOs) are involved in chemical bonding, one can expect only s-aromaticity or s-antiaromaticity.  If p-AOs are involved, s-tangential (s_t-), s-radial (s_r-), and p-aromaticity/antiaromaticity could occur.  In this case, there can be multiple (s- and p-) aromaticity, multiple (s- and p-) antiaromaticity, and conflicting aromaticity (simultaneous s-aromaticity and p-antiaromaticity or s-antiaromaticity and p-aromaticity).  If d-AOs are involved in chemical bonding, s-tangential (s_t-), s-radial (s_r-), p-tangential (p_t-), p-radial (p_r-), and d-aromaticity/antiaromaticity could occur.  In this case, there can be multiple (s-, p-, and d-) aromaticity, multiple (s-, p-, and d-) antiaromaticity, and conflicting aromaticity (simultaneous aromaticity and antiaromaticity involving s, p, and d bonds). The s- and p-aromaticity in transition metal systems have already been discussed, while the d-aromaticity has not been definitely identified yet.  In a joint experimental and theoretical study we recently have shown that the Ta₃O₃^– D_3h ¹A₁' cluster is the first cluster with the d-aromaticity (Figure 1).

Figure 1. Molecular structure  of Ta₃O₃^– D_3h ¹A₁' (a) and its upper molecular orbitals (b).

The structure and bonding in Ta₃O₃^– can be understood by analyzing its molecular orbitals (Figure 1b). Out of 34 valence electrons in Ta₃O₃^–, 24 belong to either pure oxygen lone pairs or those polarized towards Ta (responsible for the covalent contributions to Ta-O bonding).  The other ten valence electrons are responsible for the direct metal-metal bonding as shown in Figure 1b.  Among the five upper MOs, three MOs are of s-type: the partially bonding/antibonding doubly degenerate 4e' HOMO and the completely bonding 3a₁' HOMO-3.  The antibonding nature of the completely occupied doubly degenerate HOMO significantly reduces the bonding contribution of  the completely bonding HOMO-3 to the s-bonding in the Ta₃ framework.  If the HOMO (4e') and the HOMO-3 (3a₁') were composed out of the same s-d hybrid functions, bonding due to these MOs would be completely canceled.  However, the hybridization in the 4e' and 3a₁' orbitals is somewhat different.  Therefore, there should remain some s-aromatic bonding in Ta₃O₃^-.  In the Ta₃O₃^- anion, the HOMO-2 (2a₂") is a completely bonding p orbital composed primarily out of the 5d orbitals of Ta, giving rise to p-aromatic character according to the (4n + 2) HŸckel rule for p-aromaticity for ring molecules with odd numbers of atoms in the ring.  Here, we apply the (4n + 2) counting rule (odd number of atoms in the metal cycle) separately for each type of aromaticity encountered in a particular planar system, i. e. separately for s-, p-, and d-type molecular orbitals. The HOMO-1 (4a₁'), which is a completely bonding orbital mainly coming from the overlap of the d_z² orbital on each Ta atom is in fact a d-aromatic orbital. Therefore, the Ta₃O₃^– cluster possesses an unprecedented multiple (d- and p-) aromaticity, which is responsible for the metal-metal bonding and the perfect triangular Ta₃ framework.  The energy ordering of s (HOMO-3) < p (HOMO-2) < d (HOMO-1) molecular orbitals indicates that the strength of the metal-metal bonding increases from d to p to s, in agreement with the intuitive expectation that s-type overlap is greater than p-type overlap, and d-type overlap is expected to be the weakest. This work was highlighted in Chemical & Engineering News magazine (May 7, 2007, page 54).

           We also theoretically predicted that the Hf₃ cluster in the lowest singlet D_3h, ¹A₁' (1a₁'²2a₁'²1e'⁴1a₂"²3a₁'²) state possesses triple (s-, p-, and d-) aromaticity. The valence 1a₁'- and 1e'-MOs are primarily composed out of 6s-AOs of Hf and as in Ta₃O₃^- do not contribute to bonding significantly. The six valence d-electrons populate completely bonding delocalized s-MO (2a₁'), p-MO (1a₂"), and d-MO (3a₁').  The former three MOs render s-, p-, and d-aromaticity. Thus the Hf₃ cluster in the D_3h, ¹A₁' state represents the first example of a chemical system with the triple s-, p-, and d-aromaticity. We believe that transition metal systems with triple antiaromaticity and all types of conflicting aromaticity outlined above will be identified soon.

          We wrote an Invited Article on aromaticity in transition metal systems for Physical Chemistry Chemical Physics journal, in which we summarized our results as well as the results reported in the literature. We expect that aromaticity and antiaromaticity will become as commonly used in deciphering chemical bonding in transition metal systems such as carbonyls of transition metals and other known chemical compounds with transition metal clusters as these concepts are commonly used today in organic chemistry.

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