Dicarboranes generally adopt global minimum predicted by the well-known Wade–Mingos rules, although one classical non-closo structure in the benzvalene form has long been pursued and later synthesized. Here we predicted two new non-closo global minima for 6-vertex dicarboranes (C2B4R6), i.e., trigonal bipyramid (R = SH) and butterfly (R = Cl, NH2, OH, F). The long expected classical benzvalene-like structure, however, is not the global minimum for any of the nine substituents.
Carboranes, an important class of borane derivatives, obtained by replacing one or more B(H) vertexes with C(H), have been attracting considerable interest for more than half a century.1–3 Due to the “electron deficiency” of boron, the low-lying global minima of carboranes and boranes can be nicely interpreted by the well-known polyhedral skeletal electron pair theory (PSEPT) or simply Wade–Mingos rules (W–M),4i.e., species with (n + 1), (n + 2), (n + 3) PSEPs prefer the closo, nido and arachno polyhedral structures, respectively (n is the number of vertexes of the cluster). Accordingly, the six-vertex dicarboranes (C2B4H6 and derivatives), one of the central families of carboranes and the topological models of compounds containing group 13 elements,5 should take the closo form. In fact, since the first discovery of two closo structures of C2B4H6 (see A and B in Scheme 1) in 1963,6 the closo structures have been the subject of a large number of studies due to their high symmetry and unusual chemical bonding.1f,i,5d,f,7,8 Clearly, the bonding of boron in these carboranes is “non-classical” without formal vacant orbitals.
