Observation of the Highest Coordination Number in Planar Species: Decacoordinated Ta©B₁₀⁻ and Nb©B₁₀⁻ Anions

Abstract

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. [1][2][3][4][5][6][7] 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 D 7h B 8 neutral or B 8 2À as a part of the LiB 8 À cluster [1,3] or eight in the D 8h B 9 À molecular wheel.[1]The CB 6 2À , C 3 B 4 , and CB 7 À wheel-type structures with hexa-and heptacoordinated carbon atom were first considered computationally by Schleyer and co-workers. [8,9] The high symmetry hypercoordinated structures were found to be local minima because they "fulfill both the electronic and geometrical requirements for good bonding". [8,9] 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. [10,11] 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. [12][13][14] 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 AlB 6 À , AlB 7 À , AlB 8 À , AlB 9 À , AlB 10 À , and AlB 11 À systems.[ [15][16][17] Recently, a transition-metal-doped boron cluster, RuB 9 À , with the highest coordination number known to date was reported.[18] We developed a chemical bonding model, which allows the design of planar molecules with high coordination numbers.[18] According to the model, 2n electrons in the MB n 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-o...

Global optimization of clusters using electronic structure methods

“…The CK-DFT method has been used to predict the putative GM minimum for many cluster systems [202] : most recently, it was applied in conjunction with high-resolution photoelectron spectroscopy to predict the GM of pure and transition metal-doped boron cluster anions. [189,195] Besides the GA, there are many more nature inspired optimization algorithms but, to our knowledge, only PSO has been used in structure prediction of clusters, using electronic structure methods. [86,87] For example, Lv et al [87] extended their original PSO algorithm to explore the PES of clusters more thoroughly by performing several local PSOs at the DFT level of theory.…”

Sub‐Nano Clusters: The Last Frontier of Inorganic Chemistry

“…It might be appealing to view them as miniature nanoparticles or tiny chunks of the bulk solid, but beware: they have nothing in common with the structure of the extended solid or even of larger nanoparticles. Subnano clusters often have unusual structures that do not obey our intuition, such as those shown in Figure 1 Cui et al ;Galeev et al 2012;Zhai et al 2003;Zubarev and Boldyrev 2009). In their electronic structure, clusters are most reminiscent of molecules.…”

“…Additionally, also driven by double aromaticity, the central B atoms exhibit extreme coordination numbers of 7 and 8, which are unprecedented for the chemistry of boron. Since 2003, a record for coordination number in plane of 10 was made in TaB 10 -( Figure 1B), with the Ta atom residing in the center of the flat B 10 cycle (Galeev et al 2012). TaB 10 -is also doublyaromatic.…”

“…Representatives of unusual clusters: A) B 9 -is a wheel . B) TaB 10 - (Galeev et al 2012). C) Au 20 (Zubarev and Boldyrev 2009).…”

“…In fact, cluster design based on chemical bonding already has successful precedents Galeev et al 2012;Tiznado et al 2009;Zhang and Alexandrova 2013). We, as a community, are in the privileged position to write a new textbook chapter on the chemistry of clusters.…”