Far i.r. spectra are reported for 34 adducts of phosphine and amine bases with copper (I) halides in which the copper atom is coordinated to only one terminal halide. Cu---X stretching frequencies are assigned for all of the chloro complexes and for most of the bromo and iodo complexes. The Cu---X stretching frequencies have been found to depend primarily on the CuImage bond length, and appear to be relatively independent of the nature of the coordinating ligands. Best fit curves to the experimental data correspond to a dependence of ?(CuX) on the inverse nth power of r(CuX), where n is approximately equal to 5. Metal-halogen bond stretching force constants have been calculated for copper(I) and related silver(I) and gold(I) halide complexes assuming that the M---X entity behaves as an uncoupled diatomic molecule. The results show that for three-coordinate copper(I) the force constants decrease in the order Cu---Cl>Cu---Br>Cu---I and that the same trend is shown for four-coordinate copper(I) complexes, but the differences are considerably smaller than for the three-coordinate case. Analogous trends are found for the two- and three-coordinate gold compounds.