Synthesis, spectroscopic characterisation, and single-crystal structure analyses are described for a number of Cu^I and Cu^II 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 Cu^II 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 Cu^II 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 Cu^II systems. The observed ν_CO bands at ca. 2 100 cm^–1 for the Cu^I–CO adducts agree with end-on CO co-ordination.