The three-dimensional polymeric compounds [Fe(pz)M(CN)4]·nH2O (pz = pyrazine; M = Ni(II), Pd(II), and Pt(II)) have been synthesized and characterized. They undergo strong cooperative spin transitions, large hysteresis loops, and dramatic color changes upon spin conversion. The two-dimensional homologues [Fe(py)2M(CN)4] (py = pyridine; M = Ni(II), Pd(II), and Pt(II)) also have been synthesized and characterized. In the latter case cooperativity is smaller than in the tri-dimensional derivatives, and consequently narrower hysteresis loops were observed.