Identification of diborane(4) with bridging B-H-B bonds.

The irradiation of diborane(6) dispersed in solid neon at 3 K with tunable far-ultraviolet light from a synchrotron yielded a set of IR absorption lines, the pattern of which implies a carrier containing two boron atoms. According to isotope effects and quantum-chemical calculations, we identified this new species as diborane(4), B2H4, possessing two bridging B-H-B bonds. Our work thus establishes a new prototype, diborane(4), for bridging B-H-B bonds in molecular structures.