Persistent Covalency and Planarity in the B n Al 6 n 2 and LiB n Al 6 n ( n = 0 6 ) Cluster Ions

C and metallic bonding in solids profoundly impacts their properties, which have been widely exploited by humankind since ancient times. More exotic types of chemical bonding in solids are also known, for example, mixed ioniccovalent bonding characteristic of certain ceramics and perovskites. The basic classification of materials according to the type of chemical bonding in them goes together with our intuition about their structure and properties. However, it is less understood how similar covalent-metallic bonding interplay manifests itself at the small cluster size. As modern technology evolves toward nanoand subnanoscales, it becomes increasingly important to have a qualitative understanding of the chemical bonding in clusters to have intuition about them and predict their properties and shapes, much as we are capable of doing for extended solids and surfaces. Recent theoretical and experimental discoveries have moved us closer to such understanding. Among a few successful examples are clusters of boron and aluminum. 8 Despite the apparent electronic structure similarity of B ([He]2s2p) and Al ([Ne]3s3p), their clusters adopt very different structures, except at the smallest cluster size of 1 5 atoms. Specifically, all-B clusters are planar, 4 whereas all-Al clusters are closed 3-D. 8 Consider the two species, the B6 2 and Al6 2 anions, observed experimentally in the form of lithium salts, LiB6 2,3 and LiAl6 . In Figure 1, the global minimum forms of the B6 2 and Al6 2 ions are shown, along with the details of chemical bonding in them. Both clusters can be described as formed by two triangular units, B3 and Al3 . In the case of B6 2 the two units bind in plane, forming a planar D2h ( Ag) cluster, whereas for Al6 2 , the two units bind in 3-D forming an octahedron, Oh (A10). Upon coordination of Li , the shapes and nature of the chemical bonding in the ions are preserved. Detailed analysis of the chemical bonding in these clusters can be found in the literature, and here we will present only a brief overview of these findings and strategically compare the two, thus posing the question to be answered. The reason for the observed structural difference is explainable on the basis of the affordability of s p hybridization of atomic orbitals (AOs) in B versus Al. In B, the 2s 2p energy separation is smaller, and the hybridization is more attainable. This hybridization is adopted to achieve 2c 2e B B covalent bonding. Indeed, the HOMO-9, HOMO-8, HOMO-7, HOMO-6, HOMO-5, and HOMO-1 in B6 2 can be localized, as six covalent bonds along the periphery of the planar cluster. The rest of the chemical bonding in the system is realized through delocalized MOs (the HOMO, HOMO-2, HOMO-3, and HOMO-4), and the cluster was characterized as doubly antiaromatic (σ andπ) because of the population of the πand σsubsets of delocalized MOs by four electrons each (Figure 1A). In Al, the larger nuclear charge favors the AOs of lower angular momentum, the 3s 3p energy separation increases, and mixing is discouraged. As a result, in contrast with B6 2 , no directional covalent bonding occurs in Al6 2 . The HOMO-9, HOMO-8, HOMO-7, HOMO-6, HOMO-5, and HOMO-2 are simply combinations of nonoverlapping 3s AOs on Al atoms, whose net bonding effect in the cluster is zero. The entire bonding in Al6 2 thus comes from the delocalized MOs: the HOMO and HOMO-1 (Figure 1B). Because of the population of these MOs, the system was previously characterized as 3-D aromatic.