The desymmetrization of the cyclic (alkyl)(amino)carbene-supported diboracumulene B-2(cAAC(Me))(2) (cAAC(Me) = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) by monoadduct formation with IMeMe (1,3-dimethylimidazol-2-ylidene) yields the zerovalent sp-sp(2) diboron compound B-2(cAAC(Me))(2)(IMeMe), which provides a versatile platform for the synthesis of novel symmetrical and unsymmetrical zerovalent sp(2)-sp(2) diboron compounds by adduct formation with IMeMe and CO, respectively. Furthermore, B-2(cAAC(Me))(2)(IMeMe) displays enhanced reactivity compared to its symmetrical precursor, undergoing spontaneous intramolecular C-H activation and facile twofold hydrogenation, the latter resulting in B-B bond cleavage and the formation of the mixed-base parent borylene (cAAC(Me))(IMeMe)BH.