A monodimensional heterotrimetallic coordination polymer of formula ∞ 1 [{Cu II Mn II L 2 } 3 {(µ-NC) 3 W V (CN) 5 } 2 ]·H 2 O (1) has been assembled from pre-designed [Cu II Mn II L 2 ] 2+ cationic modules and homoleptic [W V (CN) 8 ] 3-metalloligands (H 2 L 2 is the compartmental Schiff-base resulting from the stepwise condensation of 2,6-diformyl-p-cresol with ethylenediamine and di-Polynuclear complexes containing different paramagnetic metal ions are of high interest in molecule-based magnetism. [1] The large majority of the heterometallic molecular systems are constructed from two different metal ions. Conversely, complexes with three different paramagnetic centers are significantly less common, mainly because of the difficulties related to the scrambling of the metal ions. [2] A rational synthetic approach towards heterotrimetallic complexes consists of assembling preformed heterobimetallic modules and metalloli-gands. [2] Anionic cyanido complexes, homo-and heteroleptic, are excellent metalloligands for building magnetic molecule-based materials containing two, or more, different metal ions. [3] Our work in this field of research has been focused on the use of heterobimetallic tectons like phenoxido-bridged 3d-3d′ and 3d-4f complexes, with the metal ions enclosed into different coordination sites generated by dissymmetric macrocyclic or [a] Dr. 3111 ethylenetriamine). The magnetic properties of 1 have been investigated revealing a three-dimensional ordered antiferromag-netic ground state below 4 K resulting from different antiferro-magnetic interactions between all the spin carriers (S = 1/2 Cu II , S = 5/2 Mn II , and S = 1/2 W V centers). side-off compartmental ligands. [4] Indeed, most of the known heterotrimetallic complexes are assembled using 3d-4f precursors , which are easily obtained by exploiting the differences between 3d and 4f metal ions [coordination numbers, HSAB-"hard and soft (Lewis) acids and bases"-behavior] and using side-off compartmental ligands. [5] The synthesis of 3d-3d′ magnetic building-blocks is more delicate, since the differences mentioned above are less prominent. One appropriate pair is composed of Cu II and Mn II ions, especially because Mn II does not have a particular stereochemical preference, while Cu II does. Robust binuclear Cu II-Mn II complexes have been generated using side-off compartmental ligands (like H 2 L 1 see Scheme 1) with an open compartment that allows various coordination numbers and geometries for the Mn II ion, while the inner compartment hosts a Cu II metal ion with a square-planar or square-pyramidal geometry (Scheme 1). [6] Scheme 1. For analogous bis-phenoxido-bridged Cu II-Mn II binuclear complex, [Cu II Mn II L 2 ] 2+ (Scheme 1), the metal ions are held together by a dissymmetric compartmental Robson-type ligand. [7] Both binuclear systems are characterized by an S = 2 ground state, which results from a significant antiferromagnetic interaction between S = 1/2 Cu II and S = 5/2 Mn II centers, and are thus good building-block candidates for designing heterotrimetallic magnetic complexes and networks. For example, the self-as