The reaction between aqueous solutions of Ni(ClO4)2, NaN3 and 2,2-dimethylpropane-1,3-diamine (Me2tn) gives blue-green crystals of the title compound which presents N3- in a new coordination mode. The new complex (C5H14N8Ni)n crystallizes in the monoclinic system, space group P2(1)/c, with a = 12.649(3) angstrom, b = 7.031(2) angstrom, c = 12.114(3) angstrom, beta = 112.39(2)degrees, V = 996.1(8) angstrom3, Z = 4, R = 0.059, and R(w) = 0.059. The Ni(II) atoms are octahedrally coordinated by two nitrogen atoms of the amine ligand, one N3 terminal ligand and three other bridging N3- ligands shared by three Ni(II) atoms, giving the stoichiometry [Ni(N3)(N3)3/3(Me2tn)]. Each bridging azido ligand coordinates two Ni(II) ions in end-on mode but, at the same time, this azido ligand coordinates the neighboring Ni(II) in an end-to-end mode. This new and unexpected coordination mode of the azido ligand stabilizes a bidimensional polymer made of mu-(1,1-N3)2Ni2 units linked together by the bridging azides, thus creating a kind of metallomacrocycle which repeats throughout the layer. Magnetic measurements from room temperature down to 60 K show a typical 2D antiferromagnetic behavior; at around 60 K an abrupt field-dependent peak of susceptibility is observed, indicating a long-range magnetic ordering with a net magnetic moment (canting). Magnetization curves at low temperature clearly confirm the presence of weak ferromagnetism. The canted spin structure is analyzed in view of the structure and the single-ion anisotropy of the Ni(II) ion.