The chemistry of lower boranes: from synthesis and structure to reactivity and applications

This review provides a systematic overview of the chemistry and properties of lower boranes (boron hydride clusters in the B₃ to B₉ range)., with emphasis on their structural features, synthesis methods, physical and spectroscopic properties, and reactivity. Although elemental boron does not react directly with hydrogen, the extensive class of boron-hydrogen compounds exhibits unique structural and reactive diversity, stemming from the electron-deficient nature of boron and the formation of multicenter bonds (e.g., three-center two-electron B–H–B and B–B–B bonds). The work systematizes synthesis methods and details the molecular structure and spectral characteristics of key representatives of this class: diborane (B₂H₆), tetraborane (B₄H₁₀), pentaboranes (B₅H₉, B₅H₁₁), hexaboranes (B₆H₁₀, B₆H₁₂), octaboranes (B₈H₁₂, B₈H₁₄, B₈H₁₈), nonaborane (B₉H₁₅), and the octahydrotriborate anion [B₃H₈]^–. For each compound, methods of preparation, molecular structure, spectroscopic data, and key physicochemical properties are discussed. The historical role of Alfred Stock and William Lipscomb in establishing this field is emphasized. On one hand, this work aims to consolidate the extensive yet fragmented knowledge on boron hydrides; on the other hand, the vastness of the field has necessitated a selective presentation of the most essential data. Although borohydrides have been known for decades, they continue to offer a rich and dynamic domain for research, with ongoing discoveries and applications in modern chemistry and materials science. We hope this review will serve as a valuable resource for researchers, students, and industrial practitioners, facilitating further exploration and innovation in this promising area.