Observation of the highest coordination number in planar species: decacoordinated Ta©B10(-) and Nb©B10(-) anions.

Coordination number is one of the most fundamental characteristics of molecular structures. Molecules with high coordination numbers often violate the octet and the 18 electron rules and push the boundary of our understanding of chemical bonding and structures. We have been searching for the highest possible coordination number in a planar species with equal distances between the central atom and all peripheral atoms. To successfully design planar chemical species with such high coordination one must take into account both mechanical and electronic factors. The mechanical factor requires the right size of the central atom to fit into the cavity of a monocyclic ring. The electronic factor requires the right number of valence electrons to achieve electronic stability of the high-symmetry structure. Boron is known to form highly symmetric planar structures owing to its ability to participate simultaneously in localized and delocalized bonding. The planar boron clusters consist of a peripheral ring featuring strong two-center-two-electron (2c-2e) B–B s bonds and one or more central atoms bonded to the outer ring through delocalized s and p bonds. The starting point for the present work is that the bare eight-atom and nine-atom planar boron clusters were found to reach coordination number seven in the D7h B8 neutral or B8 2 as a part of the LiB8 cluster or eight in the D8h B9 molecular wheel. The CB6 2 , C3B4, and CB7 wheel-type structures with hexaand heptacoordinated carbon atom were first considered computationally by Schleyer and co-workers. The high symmetry hypercoordinated structures were found to be local minima because they “fulfill both the electronic and geometrical requirements for good bonding”. In particular, Schleyer and co-workers pointed out that the wheel structures are p aromatic with 6 p electrons. In joint photoelectron spectroscopy (PES) and theoretical studies it was shown that carbon occupies the peripheral position in such clusters rather than the center, because C is more electronegative than B and thus prefers to participate in localized 2c-2e s bonding, which is possible only at the circumference of the wheel structures. A series of planar wheel-type boron rings with a main group atom in the center and coordination numbers 6– 10 have been probed theoretically. So far the joint PES and ab initio studies of aluminum-doped boron clusters showed that the aluminum atom avoids the central position in the AlB6 , AlB7 , AlB8 , AlB9 , AlB10 , and AlB11 systems. Recently, a transition-metal-doped boron cluster, Ru B9 , with the highest coordination number known to date was reported. We developed a chemical bonding model, which allows the design of planar molecules with high coordination numbers. According to the model, 2n electrons in the M Bn species form n 2c-2e peripheral B–B s bonds. The remaining valence electrons form two types of delocalized bonding, in-plane s and out-of-plane p bonding, and therefore, should satisfy the (4N+ 2) H ckel rule separately for s and p aromaticity to attain highly symmetric structures with high electronic stability. In pure wheel-type boron clusters each B atom in the circumference contributes two electrons to the B–B peripheral covalent bonds and one electron to the delocalized bonds, whereas the central B atom contributes all its valence electrons to the delocalized bonds. Thus, out of 26 valence electrons in B8 2 or 28 in B9 , 14 or 16 valence electrons form peripheral covalent 2c-2e s bonds, leaving six s and six p electrons (N= 1 for the 4N+ 2 rule) for double (s and p) aromaticity. However, pure planar boron clusters cannot go beyond coordination number eight because of the mechanical factor (the small size of the central boron atom). For example, the B10 cluster does not contain a ninecoordinated boron atom, because the boron atom is too small to fit in the central position of a B9 ring. [2] Since the central atom participates only in delocalized bonding, atoms more electronegative than boron such as carbon avoid the central position. Transition-metal atoms, on the other hand, are well-suited for the central position inM Bn species. To satisfy the peripheral B B bonding and the s and p H ckel aromaticity for N= 1, the electronic requirement for the central atom in high-symmetry species, such as M Bn k , is x= 12 n k, where x is the valence of the transition-metal atom M. Ru B9 satisfies the formula and is the first example of an [*] T. R. Galeev, Prof. Dr. A. I. Boldyrev Department of Chemistry and Biochemistry Utah State University, Logan, UT 84322 (USA) E-mail: [email protected] Homepage: http://www.chem.usu.edu/~boldyrev/