Diborane(6) as a H-bridged dimer of mono-borane can be converted cleanly by two-electron reduction into diborane (6) dianion, which is isoelectronic with ethane, through B-B sigma-bond formation when each boron atom has a bulky ligand on it. The existence of the B B sigma bond is supported by the X-ray molecular structure [B-B bond length of 1.924(3) angstrom], 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.