Electron-Deficient Triborane and Tetraborane Ring Compounds: Synthesis, Structure and Bonding.

Electron-deficient small boron rings are unique in their formation of sigma- and pi-delocalized electron systems, and the avoidance of "classical" structures in terms of two-center-two-electron (2c,2e) bonds. These rings are tolerant to several skeletal electron numbers, making their redox-chemistry highly interesting. In the past decades, the synthesis of a range of stable compounds with varying electron numbers in the B3 and B4 cores was achieved. In alliance with quantum-chemical calculations the electronic structures were evaluated. On the other hand, the chemistry of these rings is still very much underdeveloped, being generally limited to protonation and redox-chemistry of the individual systems. The linkage of several B3 and/or B4 ring systems should give compounds with attractive electronic properties, leading the way to novel boron-based materials. By summarizing important experimental and theoretical results, this article intends to provide the basis for the exploration of the chemistry of these rings, in particular their integration in larger molecular architectures.