Round and round: A mononuclear Fe(II) complex (see picture) with an N(4)S(2) coordination set has been characterized in four polymorphic forms. Two of the polymorphs display four-site cooperative spin crossover (SCO), shown conclusively by the crystal structure of a fully ordered 1:3 high-spin/low-spin state. The presence of S donor atoms in SCO-active compounds is unusual, and further investigation of Fe(II) complexes for SCO activity is warranted.