We herein present the synthesis and magneto-structural study of a new family of heterobimetallic chains of general formula {[Fe^III(dmbpy)(CN)₄]₂M^II(H₂O)₂}_n·pnH₂O [dmbpy = 4,4′-dimethyl-2,2′-bipyridine; M = Mn (2), Cu (3), Ni (4) and Co (5) with p = 4 (2), 3 (3), 9 (4) and 3.5 (5)] which were prepared by using the mononuclear PPh₄[Fe^III(dmbpy)(CN)₄]·3H₂O (1) building block (PPh₄⁺ = tetraphenylphosphonium) as a ligand toward fully solvated M^II ions. The structure of 1 consists of discrete [Fe^III(dmbpy)(CN)₄]⁻ anions, tetraphenylphosphonium cations and noncoordinated water molecules. Complexes 2–5 are isostructural compounds whose structure consists of neutral 4,2-wave like heterobimetallic chains of formula {[Fe^III(dmbpy)(CN)₄]₂M^II(H₂O)₂}_n where the [Fe^III(dmbpy)(CN)₄]⁻ entity adopts a bis-monodentate coordination mode toward trans-[M^II(H₂O)₂] units through two of its four cyanide groups in cis positions. 1 exhibits the magnetic behaviour of magnetically isolated six-coordinate low-spin Fe(III) complexes with an important orbital contribution. 2 behaves as ferrimagnetic Fe^III₂Mn^II chains, whereas 3–5 exhibit intrachain ferromagnetic couplings between the low-spin Fe(III) and either Cu(II) (3), Ni (4) or Co(II) (5) as well as frequency-dependence of the out-of-phase ac susceptibility signals below 3.0 (3), 5.5 (4) and 5.0 K (5). The relaxation time and the energy to reverse the magnetization of 3–5 are related to the anisotropy of the M(II) center and to the intra- and interchain magnetic interactions. Unprecedentedly in the world of cyanide-bearing complexes, 5 exhibits a double slow relaxation of the magnetization.