Heterocumulenes Se =CNR (R = C6H4OMe-2, C6H4Me-2) undergo facile cycloaddition to [(H-dpp-bian)Na(Et2O)(2)] (1) (H-dpp-bian = N-protonated 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) resulting in cycloadducts [(H-dpp-bian)Na(SeCNR)(DME)] (2, 3), which are the first cycloadducts derived from a sodium metallacycle reported so far. A comparative reaction of [(dpp-bian)Mg(THF)(3)] (10) with Se-C-NR gives magnesium cycloadducts [(dpp-bian)Mg(SeCNR)(Solv)(2)] (11, 12), which undergo fast decomposition at room temperature. New compounds are characterized by NMR, EPR, and IR spectroscopy, and elemental and X-ray diffraction analysis. Their electronic structures and reaction pathways were probed using DFT calculations.