Hunting dimers

HBCBH and HAlCAlH are related electrondeficient molecules, predicted as local minima on their respective potential energy surfaces. Different views of their electronic structure—as allenes, carbenes, or carbones—prompted two research groups to a friendly competition exploring the predilection of these small molecules to dimerize. Many such dimers emerged from the calculations, some with large enthalpies favoring dimerization. Most interesting among these is an octahedral didehydrocarborane (or alane). The menagerie of (HECEH)2, E = B, Al structures obtained shows great variety in its bonding patterns. That variety is instructive, for it points to the necessity of examining dimerization for any theoretically postulated metastable molecule. We also learn of the continuing utility of simple Lewis structures, the acid–base reactivity of carbones and carbenes, and the dominant role of electron deficiency in borane and alane chemistry.