Boron monoxide (BO) is a binary compound of boron and oxygen. The material was first reported in 1940, with a modified synthetic procedure published in 1955, however, the material's structure had remained unknown for nearly a century. A number of allotropes of BO have been theorized ranging from molecular species, to 1D, 2D, and 3D-structured materials, but these were difficult to differentiate using common structural characterization methods. The material sheets composed of O-bridged B<sub>4</sub>O<sub>2</sub> rings, and most likely adopts the 1D polymeric structure initially proposed in 1955. Due to the lack of precise structural information on the identity of the compound, it has not found widespread use in industry.
Boron monoxide is typically produced through the condensation of tetrahydroxydiboron (chemical formula; B<sub>2</sub>(OH)<sub>4</sub>) at temperatures of 200âÂÂ500ðC. The use of higher temperatures (700ðC) leads to the formation of hard B<sub>2</sub>O<sub>3</sub> glasses. These glasses generally have a dark appearance, from the dissolved elemental boron, and are also produced directly through the dissolution of B into B<sub>2</sub>O<sub>3</sub>.
BO has been used in the synthesis of B<sub>2</sub>Cl<sub>4</sub>, which served as the only evidence, until 2010, of the preservation of the BâÂÂB bond present in the precursor compound.