The syntheses, structures, and magnetic properties of the trinuclear triangular compounds [{CuIIL1}2Cu II(H2O)2](ClO4)2. H 2O (1) and [{CuIIL1}2M II(H2O)2](ClO4)2 {M = Ni (2), Co (3), Fe (4)} derived from N,N'-ethylenebis(3-methoxysalicylaldimine) (H2L1) are described. Compound 1 crystallizes in the monoclinic P21/n space group, while the crystal system for the isomorphous compounds 2-4 is monoclinic P21/c. In the cations of these compounds, two copper(II) ions occupy the salen type N2O 2 cavity of two [L1]2 ligands, while one phenoxido and one methoxy oxygen atom of each of these two [Cu IIL1] moieties coordinate to a central metal ion, which results in the formation of the trinuclear CuII2M II systems in which the two pairs of central-terminal metal ions are monophenoxido-bridged. In addition, to the two phenoxido bridges, the central metal ion is coordinated to two methoxy and two water oxygen atoms. The coordinated water molecules are strongly hydrogen bonded to the phenoxido and methoxy oxygen atoms. The presence of the coordinated water molecules and the interaction of these water molecules with the phenoxido and methoxy oxygen atoms are proposed to be the governing factors for tho stabilization of the rare; example of the monophenoxido-bridged CuII3 compound 1 and of the first examples of the monophenoxido-bridged CuII 2MII compounds 2-4. The three metal ions define an isosceles triangle. The phenoxido bridge angles in those compounds are significantly obtuse and vary between 121.10(12)° and 129.84(13)°. On the other hand, the trinuclear cores are highly twisted as evidenced from the range of the dihedral angles (58.5-143.7°) between the basal planes of the terminal and the central metal ions. The variable-temperature (2-300 K) magnetic susceptibilities of compounds 1-4 have been measured. The magnetic data for 1 have been analyzed by using a model of two exchange integrals for the two pairs of adjacent metal ions, while one J value is considered for the other three complexes. The exchange integrals obtained are: CuII3 compound 1: J1 = -97.6 cm-1, J2 = -89.5 cm -1; CuII2NiII compound 2: J = -25.4 cm-1; CuII2CoII compound 3: J = -9.0 cm-1 CuII2FeII compound 4: J = -6.5 cm-1. These moderate to weak antiferromagnetic interactions in 1-4 are related to the twisting of the trinuclear cores. The relative order of the exchange integrals can been understood from the nature of the magnetic orbitais. The structural and magnetic results have been compared with previously reported related compounds. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.