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Laboratory measurements with the SIFT technique have shown that magnesium radical cations in a helium bath gas at 0.35 Torr and 294 K are unreactive with hydrogen cyanide but initiate a chemical sequence in cyanoacetylene that leads to the formation of Mg(HC3N)n+• cluster ions with n up to 7. Rate‐coefficient measurements for sequential addition of cyanoacetylene to Mg+• indicate an extraordinary pattern in alternating chemical reactivity while multiple‐collision induced dissociation experiments revealed an extraordinary stability for the Mg(HC3N)4+• cluster radical cation. Molecular orbital calculations with density functional theory (DFT) have provided structures and energies for the observed Mg(HC3N)1–4+• cations. These calculations indicate that the path of formation of Mg(HC3N)4+• appears to involve ligand‐ligand interactions mediated by Mg+• to form 2,4,6,8‐tetracyanosemibullvalene‐Mg+• or 1,2,5,6‐tetracyano‐1,3,5,7‐cyclooctatetraene‐Mg+• cations. A case is made for the formation of similar complex ...