Three new 1D cyanido-bridged 3d-4f coordination polymers, {[Gd(L)(H₂O)₂Fe(CN)₆]·H₂O}n (1_GdFe), {[Dy(L)(H₂O)₂Fe(CN)₆]·₃H₂O}_n (₂DyFe), and {[Dy(L)(H₂O)₂Co(CN)₆]·H₂O}_n (3_DyCo), were assembled following the building-block approach (L = pentadentate bis-semicarbazone ligand resulting from the condensation reaction between 2,6-diacetyl-pyridine and semicarbazide). The crystal structures consist of crenel-like Ln^III-M^III alternate chains, with the Ln^III ions connected by the hexacyanido metalloligands through two cis cyanido groups. The magnetic properties of the three complexes have been investigated. Field-induced slow relaxation of the magnetization was observed for compounds 2_DyFe and 3_DyCo. Compound 3DyCo is a new example of chain of Single Ion Magnets.