The 1 : 2 adducts of copper(I) halides with 1,2-bis(2-pyridylphosphino)ethane (d2pype) have been synthesized and solution properties characterized by variable temperature 1H, 31P and 65Cu NMR spectroscopy. Single-crystal structure determinations for the chloride, bromide and iodide complexes show these to crystallize from acetonitrile in the triclinic space group P as isostructural centrosymmetric dimers [(d2pype)Cu(μ-d2pype)2Cu(d2pype)]X2·(solvent) with a 12.6, b 12.7, c15.3 Å, α 84, β 67, γ 84°. In contrast to the analogous AuCl:2(d2pype) and AgNO3:2(d2pype) adducts, in solution these CuX:2(d2pype) adducts (where X = Cl, Br and I) exist almost exclusively as bis-chelated monomeric [Cu(d2pype)2]X; evidence for an equilibrium between monomeric and dimeric forms is detected only for the CuCl adduct in methanol. Cytotoxicity studies in two human breast cancer lines and two matched liver progenitor cell lines indicate that [Cu(d2pype)2]Cl is non selectively toxic to both non-tumourigenic and tumourigenic cells. However, the analogous Au(I) compound [Au(d2pype)2]Cl, is toxic to highly tumourigenic cells and more selective in its toxicity to tumourigenic cells compared to non-tumourigenic cells. The significance of these results to the further development of selective, mitochondria-targeted, Au(I) antitumour complexes is discussed.