Electron precise boron-containing three-membered rings

Electron precise boron-containing three-membered rings represent a distinctive class of strained compounds that combine pronounced angle strain with the intrinsic electron deficiency of boron. In contrast to boron clusters stabilized by multicenter bonding, these systems feature localized two-center–two-electron skeletal bonds. Over the past decades, substantial progress has been made in the synthesis, isolation, and characterization of such rings, including homonuclear triboron frameworks as well as heterocycles incorporating carbon, silicon, tin, nitrogen, phosphorus, chalcogens, and mixed main-group elements. Beyond their structural diversity, electron precise boron three-membered rings display rich and often unexpected reactivity that arises from the interplay of ring strain and vacant boron p-orbitals. In particular, ring enlargement reactions have emerged as powerful transformations, enabling the efficient construction of larger boron-containing heterocycles, and the σ-complex/π-complex duality observed in boriranes provides a new conceptual perspective for such small-ring systems. This Review provides a systematic overview of electron precise boron-containing three-membered rings, organized by ring composition. For each class, synthetic strategies, structural and electronic features, reactivity patterns, and relevant computational insights are summarized. Special emphasis is placed on ring enlargement reactions as a common reactivity pattern. • Electron precise boron-containing three-membered rings are systematically classified. • Synthesis, structure, bonding, and reactivity across diverse BXY ring systems are summarized. • Ring enlargement emerges as a unifying strain-driven reactivity motif. • Metallomimetic behavior and π-complex character of boron small rings are highlighted. • Perspectives on the integration of boron-containing small rings into functional contexts are provided.