The crystal structure and magnetic characterization of an isostructural series of general formula {[Ln(crot)₃(H₂O)(bpy)_1/2]₂}_n (crot, crotonate (C₄H₅O₂); bpy, 4,4′-bipyridine (C₁₀H₈N₂); Ln, Nd, Gd, Ho, Er, Y) is presented. The ninefold Ln coordination polyhedra form dimeric entities that are connected throughout the bpy units into infinite polymeric chains. All (but the yttrium) reported compounds present a weak antiferromagnetic interaction connecting metal centres.