Metal complexes of the new ligand 1,2,6,7-tetracyano-3,5-diimino-3,5-dihydropyrrolizinide, M(II)(C11N7H2)2, as basic units for molecular materials. Electrochemical and metal-phthalocyanine-like electronic properties of the nickel(II) complex and x-ray crystal structures of its adducts with dioxane and tetrathiafulvalene