Boron–Boron σ-Bond Formation by Two-Electron Reduction of a H-Bridged Dimer of Monoborane

Diborane(6) as a H-bridged dimer of monoborane can be converted cleanly by two-electron reduction into diborane(6) dianion, which is isoelectronic with ethane, through B–B σ-bond formation when each boron atom has a bulky ligand on it. The existence of the B–B σ bond is supported by the X-ray molecular structure [B–B bond length of 1.924(3) Å], NMR studies, magnetic susceptibility measurements, and DFT calculations. Stepwise hydride abstraction reactions of the diborane(6) dianion produce the corresponding H-bridged diborane(5) anion and doubly H-bridged diborane(4) without B–B bond scission.