The reaction of [Cu(CH3CN)(4)](BF4) with 3,5-dimethyl-4-nitropyrazole (Hdmnpz) in the presence of triethylamine yields the new copper(I) complexes [Cu(dmnpz)](3) (1) and (Et3NH)(2)[Cu-4(dmnpz)(6)] (2), depending on the experimental conditions. The reactivity of 1 and 2 toward neutral ligands such as triphenylphosphine, cyclohexyl isocyanide (RNC), and carbon monoxide has been investigated. In particular, both complexes readily react with RNC, giving the dinuclear complexes [Cu(dmnpz)(RNC)](2) (4) and [Cu(dmnpz)(RNC)(2)](2) (5), depending on the copper/RNC ratio, and with PPh3, affording the dimeric derivative [Cu(dmnpz)(PPh3)](2) (6). The crystal and molecular structure of 1 has been determined ab initio using X-ray powder diffraction data from conventional laboratory equipment. Crystals of 1 are monoclinic, C2/c, a = 20.057(2) Angstrom, b = 13.816(2) Angstrom, c = 7.883(1) Angstrom, beta = 95.912(4)degrees; R-F and R-wp 0.067 and 0.039, respectively, for Rietveld refinement on 3900 data points collected in the range 17 < 2 theta < 95 degrees (Cu K alpha radiation). Crystals of 1 contain planar trimers, with the copper atoms bridged by exo-bidentate ligands and short intermolecular Cu ... Cu contacts (3.329(7) Angstrom). For complexes 2 and 4-6, single-crystal X-ray diffraction studies have been performed. Crystals of 2 are monoclinic, P2(1)/c, a = 11.026(1) Angstrom, b = 13.456(2) Angstrom, c = 18.668(5) Angstrom, beta = 92.91(2)degrees, Z = 2. The ionic packing of 2 contains tetranuclear complexes, with the copper atoms connected by six dmnpz ligands. Crystals of 4 are triclinic, P (1) over bar, a = 7.418(1) Angstrom, b = 9.780(1) Angstrom, c = 11.177(3) Angstrom, alpha = 109.61(2); beta = 101.34(3)degrees, gamma = 105.82(1)degrees, Z = 2. Crystals of 5 are triclinic, P (1) over bar, a = 9.506(4) Angstrom, b = 9.957(2) Angstrom, c = 11.658(4) Angstrom, alpha = 86.73(2)degrees, beta = 79.54(2)degrees, gamma = 82.47(4)degrees, Z = 1. Crystals of 6 are triclinic, P (1) over bar, a = 11.379(2) Angstrom, b = 13.592(2) Angstrom, c = 14.409(3) Angstrom, alpha = 81.22(1)degrees, beta = 85.21(1)degrees, gamma = 87.55(1)degrees, Z = 2. 4, 5, and 6 are binuclear complexes bearing one RNC, two RNC, and one triphenylphosphine ligands on each copper atom, respectively. The steric requirement of the dmnpz ligands, in the presence of RNC or PPh3, forces the inner [Cu-2(dmnpz)(2)] core of the three complexes into markedly different conformations.