Using reversible polymerization and depolymerization reactions in a single crystal state, we achieved a reversible transformation from a nanocluster to a coordination polymer. During the interconversion, the structural frameworks switched between nonporous hexanuclear clusters and porous double-zigzag chains; the magnetic behaviors switched between paramagnetism and metamagnetism, respectively. The microporous framework, which had 1D channels 1.9 A x 3.6 A in size, exhibited selective gas adsorption of H(2) and CO(2) over N(2).