A compound with a boron-boron triple bond is shown to undergo stepwise hydroboration reactions with catecholborane to yield an unsymmetrical hydro(boryl)-diborene and a 2,3-dihydrotetraborane. Abstraction of H- from the latter compound produces an unusual cationic, planar tetraborane with a hydrogen atom bridging the central B-2 moiety. Spectroscopic and crystallographic data and DFT calculations support a "protonated diborene" structure for this compound, which can also be accessed via direct protonation of the corresponding diborene.