Reversible single crystal-to-single crystal transformations were observed for the Ni(II) complex with 5-methyl-5-phenylhydantoin. Moreover, the symmetry changes from triclinic (P) to monoclinic (P2₁/c). The change is induced by a disturbance of the ammonia–water equilibrium between the [solid state–solution-gas] phases, which causes an exchange of the solvent in the crystal structure.