Facile Access to Unprecedented Electron-Precise Monohydrodiboranes(4), cis-1,2-Dihydrodiboranes(4), and a 1,1-Dihydrodiborane(5).

2,3-Bis(dimethylamino)-substituted B2 N2 C2 heterocycles underwent selective dimethylamino/hydride exchange with either one or two equivalents of BH3 ⋅SMe2 to give the corresponding cyclic monohydrido- or (cis)1,2-dihydridodiboranes(4), respectively. Upon either heating or irradiation in solution, the latter underwent ring contraction to the corresponding five-membered BN2 C2 heterocycles, whereas irradiation of the 1,2-dimethylaminoethene-supported 1,2-dihydridodiborane(4) in the presence of PEt3 gave an unprecedented unsymmetrical 1,1-dihydrodiborane(5) phosphine adduct.