Copper(I) cyanide is an important reagent in organic, organometallic, and supramolecular chemistry because of both the copper center and the versatile cyanide ligand. Solid-phase CuCN and many of its derivatives show oligomeric or polymeric structures, a trait shared by other metal cyanides. Often, it is difficult to specify the orientation of the cyano ligand in an X-ray structure. Here the first preparation and precise structure of a monomeric transition metal cyanide is reported. Gas-phase reaction between copper vapor and cyanogen (NCCN) clearly gives CuCN (not CuNC). The precise structure of CuCN so produced is determined by millimeter/submillimeter-wave spectroscopy. Because of the highly efficient synthesis and the presence of significant amounts of two copper isotopes, such strong signals were seen that natural-abundance materials allowed observation of transitions for the four isotopomers (63)Cu(12)C(14)N, (65)Cu(12)C(14)N, (63)Cu(13)C(14)N, and (63)Cu(12)C(15)N and the determination of r(o), r(s), and r(m)((2)) structures. All data unequivocally show a linear geometry and that the carbon of cyanide is bound to copper with a Cu-C distance of 1.82962(4) A in the r(m)((2)) structure, which is likely to be closest to the equilibrium geometry.