The Cu^II ions usually have different coordinated geometry to other 3d ions, especially Ni^II and Co^II ions, in azido–carboxylate mixed ligand systems. That provides a potential way to synthesize rare heterometallic 3d–3d azido complexes with peculiar magnetic properties. Assembling Cu^II, M^II (M = Ni and Co), azido and nicotinic acid in hydrothermal condition, two novel isomorphic 3D heterometallic 3d–3d azido complexes, [CuM(N₃)₂(nicotinate)₂]_∞ (M = Ni^II for 1 and Co^II for 2) were obtained. The structure of the complexes can be described as EO azido and syn,syn carboxylate mixed bridged spin sequence chains (–1/2–J₁–1–J₂–1/2– for 1 and –1/2–J₁–3/2–J₂–1/2– for 2) linked by the pyridyl groups. Dominant ferromagnetic interactions were observed between the Cu^II and M^II ions in the chains despite the largest M–N–M angle of about 129° in the 3d metal azido–carboxylate mixed coordinated systems and weak antiferromagnetic interactions between the chains. At low temperature, a spin–flop phase transition was present in the Co^II involved complex 2.