9-Borabicyclo[3.3.1]nonane (9-BBN) dimer is a stable, crystalline dimeric form of the organoborane reagent 9-BBN. It is a highly versatile and selective hydroborating agent, widely recognized for its steric bulk and regioselectivity. The compound exists as a dimer in the solid state and in nonpolar solvents, dissociating into the reactive monomer upon use. Its primary and most significant application is in organic synthesis for the hydroboration of alkenes and alkynes. 9-BBN exhibits exceptional regioselectivity, favoring anti-Markovnikov addition to give the less substituted alkylborane. This selectivity is particularly valuable for the synthesis of primary alcohols from terminal alkenes and for the preparation of organoboron intermediates used in subsequent cross-coupling reactions (e.g., Suzuki-Miyaura coupling). It is also employed as a catalyst or catalyst precursor in various organic transformations, including reductions and isomerizations. Due to its air- and moisture-sensitive nature, it is typically handled under inert atmosphere (argon or nitrogen). It is commercially available as a solution in tetrahydrofuran or as a solid dimer, offering convenience for different synthetic protocols. Its predictable reactivity and selectivity make it a fundamental tool in research and process chemistry for constructing complex molecules.