Synthesis, spectroscopic characterisation, and single-crystal structure analyses are described for a number of CuI and CuII co-ordination compounds with the ligand 1,8-bis[bis(1′-methyl-benzimidazol-2′-ylmethyl)amino]-3,6-dioxaoctane (L). The compounds have the general formulae Cu2(L)X2Y2·xS, Cu(L)Y, Cu2(L)(CO)2Y2, Cu2(L)(Z)Y3, and Cu2(L)X2, where X = F, Cl, Br, or I; Y = ClO4, BF4, CF3SO3, Cl, or Br; Z = bridging anion like benzotriazolate; and S = solvent. Single-crystal structures of three CuII compounds, [Cu2(L)F2][BF4]2·2H2O·H2OBF3(1), [Cu2(L)Cl2]Cl2·6H2O (2), and [Cu2(L)Br2][CF3SO3]2·EtOH (3) have been determined using diffractometer data. All three compounds contain the dinuclear unit [XCu(L)CuX]2+ in which the ligand L bridges the two Cu–X fragments. However, the intramolecular Cu ⋯ Cu distances are very large (i.e. larger than 575 pm). The intermolecular Cu ⋯ Cu contacts are much shorter, being 481, 388–488, and 398–405 pm, for (1), (2), and (3), respectively. Ligand-field, e.s.r., n.m.r., and i.r. spectra have been used to characterise all the compounds. The ligand-field maxima all agree with square-pyramidal CuII with a CuN3XO chromophore in which X = F, Cl, or Br, respectively. The e.s.r. spectra indicate that exchange effects (intermolecularly) are important, resulting even in exchange-narrowed signals. Only the compounds Cu2(L)(Z)Y3 exhibit e.s.r. spectra typical for dinuclear CuII systems. The observed νCO bands at ca. 2 100 cm–1 for the CuI–CO adducts agree with end-on CO co-ordination.