Electron‐deficient triborane and tetraborane ring compounds: Synthesis, structure, and bonding

Electron‐deficient small boron rings are unique in their formation of σ‐ and π‐delocalized electron systems as well as the avoidance of "classical" structures with two‐center‐two‐electron (2c,2e) bonds. These rings are tolerant of several skeletal electron numbers, which makes their redox chemistry highly interesting. In the past few decades, a range of stable compounds have been synthesized with various electron numbers in their B${}_3$ and B${}_4$ cores. The electronic structures were evaluated by quantum‐chemical calculations. On the other hand, the chemistry of these rings is still very much underdeveloped, being generally limited to the protonation and redox reactions of individual systems. The linkage of several B${}_3$ and/or B${}_4$ ring systems should give compounds with attractive electronic properties, thus leading the way to novel boron‐based materials. By summarizing important experimental and theoretical results, this Review intends to provide the basis for the exploration of the chemistry of these rings and, in particular, their integration into larger molecular architectures. (© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)