The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both σ and π delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B3(+/-), B4(2-), B5(+/-), B6, B7(-), B8(2-), B9(-), B10(2-), B11(-), B12, B13(+), B14(2-) and B16(2-) are characterized by magnetic responses to be doubly σ and π aromatic species in which the π aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B12 and B13(+) is confirmed. The π electrons of B18(2-), B19(-) and B20(2-) obey the disk aromaticity rule with an electronic configuration of [1σ(2)1π(4)1δ(4)2σ(2)] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B5H5(+), Li7B5H5 and M@BnHn(q) clusters from both the (4N + 2) rule and ring current maps. The double π and σ aromaticity in carbon-boron planar cycles B7C(-), B8C, B6C2, B9C(-), B8C2 and B7C3(-) is in conflict with the Hückel electron count. This is also the case for the ions B11C5(+/-) whose ring current indicators suggest that they belong to the class of double aromaticity, in which the π electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character.

A well-defined class of planar or quasi-planar elongated boron clusters, of type B q2 713n , serves as a basis to identify the valence bond picture of delocalized boron networks. The origin of the series is the B 2 7 cluster, which exhibits r-aromaticity. The cluster generating step is the repetitive expansion by three boron atoms in the direction of elongation. Specific electron counting rules are obtained for p-bonding, peripheral r-bonding and multicenter inner r-bonding. A valence bond structure is introduced which explains the remarkable regularity in the bonding pattern. The analysis supports 4c-2e bonds as an alternative to the common 3c-2e bonds. The results are validated by symmetry induction and ab initio calculations. K E Y W O R D S boron clusters, computational chemistry, induction method, multicenter bonding 1 | I N TR ODU C TI ONThe recent literature reports on a wide variety of planar and bowl shaped boron clusters. Proposed structures are usually based on theoretical calculations, but for some cases structures could be confirmed by photoelectron or infrared spectroscopy [1] on clusters produced by laser evaporation. [2][3][4] A further special feature of some clusters which are shaped like two concentric rings is the almost barrierless rotatory motion of the inner ring with respect to the outer ring. [5] This motion has been compared to a Wankel motor at the molecular scale. [6] In view of the rich variety of shapes and properties, which challenges accepted concepts of chemical bonding, boron is said to be the new carbon. As opposed to carbon, it is known to adopt multicenter bonds which have to be accomodated in a proper theoretical scheme. [7][8][9] To build a consistent valence bond picture that would apply to all these clusters, a gradual approach is required based on a well-defined set of structures. For this, we chose the particular family of the so-called elongated boron clusters. The aim is to obtain a set of rules that rationalize the electronic structure calculations on a series of structures extending from B 2 7 to B 22 28 . [10][11][12][13][14][15][16][17][18] Int J Quantum Chem. 2018;118:e25575.