A novel metal complex of a heterocyclic aromatic compound which shows a low activation energy, is stabilized structurally, is capable of modulation of the structure thereof, and capable of preferably functioning as a molecular device in technological fields. The metal complex of a heterocyclic aromatic compound comprises a transition metal (for example, silver ion) as a central atom, and basic ligands comprised of a 5-membered heterocyclic aromatic compound (for example, pyrrole rings), in which the position of the central atom can be changed by an internal factor such as transfer of an electric charge or by an external factor such as application of an electric field, a change in acidity of the surrounding environment, etc., whereby the number of atoms (or the number of electrons) relating to the coordination ability can be modulated, and, upon polymerization, the conformation can be modulated depending on the position of the central atom.