The synthesis, single-crystal structures and magnetic properties of three new cyano-bridged complexes [Fe^III₄M^II₂] (M = Fe^II, Co^II, Ni^II) are reported. Electronic absorption spectroscopy reveals the occurrence of intervalence charge transfer for [Fe^III₄Fe^II₂]. Field-induced slow magnetic relaxation behavior is exhibited for [Fe^III₄Co^II₂] and [Fe^III₄Ni^II₂] which originates from the ferromagnetic interactions between Fe^III and M^II spin centers of the square.