Dinuclear [(hfac)Cu](2)(MHY) and [(hfac)Cu](2)(HY) species, where hfac = hexafluoroacetylacetonate, MHY = 2-methyl-1-hexen-3-yne, and HY = hex-3-yn-1-ene), are formed during copper chemical vapor deposition experiments when a bubbler containing the corresponding mononuclear species is used. These compounds and the so far unknown (hfac)Cu(HY) have been characterized by H-1 and C-13 NMR and IR spectroscopy and the two dinuclear species by X-ray crystallography and elemental analysis. As expected, in the structure of the dinuclear species, two roughly perpendicular Cu(hfac) moieties are bound to a single ene-yne: one to the C-C triple bond, the other to the conjuguated double bond. Weak intra- and intermolecular Cu . . . Cu interactions have been detected in the solid. [(hfac)Cu](2)(HY) is more stable and more easily formed than [(hfac)Cu](2)(MHY) because of the hindrance character of the additional methyl groups. The formation of the dinuclear species from the mononuclear species is reversible as it has been demonstrated by 1H NMR. The mononuclear species can be recovered from the dinuclear species when,free ene-yne base is added. Crystallographic data for [(hfac)Cu](2)-MHY): C17H12O4F12Cu2, triclinic, P (1) over bar (No. 2), Z = 4; at 298 K, a = 11.112(3) Angstrom, b = 13.472(4) Angstrom, c = 15.747(3) Angstrom, alpha = 94.95(2)degrees, beta = 98.15(2)degrees, gamma = 100.27(2)degrees; Crystallographic data for [(hfac)Cu](2)(HY): C16H10O4F12Cu2, orthorhombic, Cc2a (No. 41, Aba2), Z = 8; at 230 K, a = 12.280(3) Angstrom, b = 13.531(3) Angstrom, c = 25.461(5) Angstrom.