By following a similar procedure to that employed for the synthesis of [HP7]2– (1), the synthesis and characterization of the hydrogenheptaarsenide dianion [HAs7]2– (2) was achieved. Building on previous research that has shown that [HP7]2– can react with carbodiimides to yield amidine-functionalized cluster anions [P7C(NHR)(NR)]2–, we have found that the analogous hydroarsination reactions are also possible by reaction of 2 with RC=N=CR to yield [As7C(NHR)(NR)]2– [R = Dipp (3), iPr (4) and Cy (5); Dipp = 2,6-diisopropylphenyl]. Furthermore, we also demonstrate that such hydropnictination reactions can be extended to other heteroallenes such as isocyanates (O=C=NAd; Ad = adamantyl) to afford the amide-derivatized cluster anions [E7C(CO)(NHAd)]2– [E = P (6) and As (7)]. All new compounds were characterized by multinuclear NMR spectroscopy, elemental analyses and electrospray mass spectrometry. Clusters 2, 4, 6 and 7 were also characterized by single-crystal X-ray diffraction of [K(18-crown-6)]2, [K(18-crown-6)]2, [K(18-crown-6)]2·py (py = pyridine) and [K(18-crown-6)]2·py. Crystal structures confirming composition and connectivity were also obtained for anions 3 and 5.