The hydrothermal reaction of NaN₃ with Cu(Phen)Cl₂ in acetonitrile solutions results in the formation of two complexes: new d⁹ mononuclear cis-[Cu^II(Phen)₂(mtz)₂]·H₂O and d¹⁰ coordination polymer catena-[Cu^I(Phen)(μ-CN)]_n (mtz is 5-methyltetrazolate anion and Phen is 1,10-phenanthroline). The process involves in situ formation of mtz ligand via cycloaddition of acetonitrile and azide (in the case of [Cu^II(Phen)₂(mtz)₂]) and cleavage of acetonitrile C–C bond (in the case of [Cu^I(Phen)(μ-CN)]_n). Both complexes were fully characterized by a comprehensive set of methods, including the single crystal X-ray diffraction data.