Ternary copper(i) complexes with alkynes and 1,10-phenanthroline (phen) have been prepared and characterized by their formation constants, IR spectra, H-1 and C-13 NMR spectra, The structures of three complexes, [Cu(phen)(HC=CR)]ClO4 (R = H 1, Ph 2 or CO2Et 3) have been determined using single-crystal X-ray methods. Each copper atom in all three complexes is co-ordinated to two nitrogen atoms of phen and an alkyne molecule in a trigonal-planar arrangement. The alkyne is sideways bonded onto the copper atom. The C=C bond distances of 1.190(7) (1), 1.218(13) (2) and 1.193(10) angstrom (3) are slightly longer than those of the free alkynes and indicate that pi-back donation from copper to the alkyne is weak. The reduction of the C=C stretching frequency for metal-alkyne complexes is correlated to the acetylenic bend-back angles and the lengthening of the C=C distance of the alkyne upon co-ordination. The copper-carbon and -nitrogen distances for 1 and 2 are essentially the same as those for the corresponding olefin complexes.