The work in this paper aims to portray a complete structural, magnetic, and theoretical description of two original endto-end (EE)μ1,3-azide- bridged, cyclic tetranuclear NiII clusters, [{NiII(L 1)(M1,3-N3)(H2O)}4] (1) and [{NiII(L2)(μ1,3-N3)(H 2O)}4] (2), where the ligands used to achieve these species, HL1 and HL2, are the tridentate Schiff base ligands obtained from [1 + 1] condensations of salicylaldehyde with 1-(2-aminoethyl)-piperidine and 4-(2-aminoethyl)-morpholine, respectively. The title compounds, 1 and 2, crystallize in a monoclinic P21 space group. Overall, both species can be described in a similar way; where all NiII centers within each molecule are hexacoordinated and bound to [L1]- or [L2]- through the phenoxo oxygen, imine nitrogen, and piperidine/morpholine nitrogen atoms of the corresponding ligand. The remaining coordination sites are satisfied by one molecule of H2O and two nitrogen atoms from N3- anions. The latest act as bridges between NiII ions, and eventually, only four azido groups are linked to the same number of NiII centers resulting in the formation of cyclic NiII4 systems. Interestingly, compounds 1 and 2 are the two sole examples of tetranuclear clusters generated exclusively by EE azide-bridging ligands to date. All the N(azide)-Ni-N(azide) moieties are almost linear in 1 and 2 indicating trans arrangement of the azido ligand. Variable-temperature (2-300 K) magnetic susceptibilities of 1 and 2 have been measured under magnetic fields of 0.04 T (from 2 to 30 K) and 0.7 T (from 30 to 300 K), and magneto-structural correlations have been performed. Despite the presence of both ferromagnetic and antiferromagnetic interactions in both compounds, significant differences have been observed in their magnetic behaviors directly related to the arrangement of the bridging azido ligands. Hence, compound 1 has an overall moderate antiferromagnetic behavior due to the presence of an exchange pathway with an unprecedented Ni-N · · · N-Ni torsion angle close to 0°, meanwhile complex 2 exhibits a predominant ferromagnetic behavior, with torsion angles between 50 and 90°. Density functional theory calculations have been performed to provide more insight into the magnetic nature of this new family of NiII-azido complexes and also to corroborate the fitting of the data. © 2010 American Chemical Society.