The synthesis, characterization and properties of the dinuclear nickel(II) complexes [{Ni-2(Medpt)(2)(N-3)(2)}(mu-(1,1-N-3)(2))] (1), [{Ni-2(dpt)(2)(N-3)(2)}(mu-(1,1-N-3)(2))] (2) and [{Ni-2(dpt)(2)(N-3)(2)}(mu-(1,3-N-3)(2))] (3), in which Medpt is methylbis(3-aminopropyl)amine and dpt is bis(3-aminopropyl)amine are described. The magnetic susceptibility for all three compounds has been recorded between 300 and 4 K; 1 and 2 show strong ferromagnetic coupling, and 3 shows antiferromagnetic behavior. Compound 1 shows a magnetic transition between 150 and 120 K. The crystal structure of 1 has been solved at 293 K and, after cooling to 118.5 K, at 190.5(3) K. Complex 1 crystallizes in the triclinic system, space group P (1) over bar, with fw = 576.0, 293 K [190.5 K], a = 7.331(1) [7.303(2)] Angstrom, b = 8.091(1) [8.056(2)] Angstrom, c = 11.389(2) [11.310(3)] Angstrom, alpha = 71.96(2) [71.89(3)]degrees, beta = 83.92(1) [83.57(2)]degrees, gamma = 73.07(1) [72.74(2)]degrees, V = 614.4(3) [603.8(5)] Angstrom(3), Z = 1, R = 0.063 [0.067], and R(W) = 0.064 [0.068]. The phase transition and the magnetic behavior of 1 are described as the result of a reversible second-order Jahn-Teller effect.