The crystal structures of the adducts of Cu₂(OCOCH₃)₄ with methanol (1), acetic acid (2), dimethylformamide (3), and 1,4-diazabicyclo [2.2.2] octane (4) have been determined using single-crystal X-ray diffraction methods. The compounds are isostructural with that of Cu₂(OCOCH₃)₄·2H₂O with the water molecules in the axial positions being replaced by the respective ligands. The ligand 1,4-diazabicyclo[2.2.2]octane bridges two dimer units producing a chain structure, in which this ligand is disordered around the N–N axis. Crystal data : (1), monoclinic, space group P2₁/n, a= 8.129(2), b= 7.447(1), c= 13.332(1)Å, β= 92.21(1)°, and Z= 2; (2), space group, P2₁/n, a= 15.153(2), b= 7.772(1), c= 8.229(1)Å, β= 103.08(1)°, and Z= 2; (3), triclinic, space group P, a= 8.002(1), b= 8.140(1), c= 9.394(2)Å, α= 105.85(2), β= 92.31 (2), γ= 113.84(1)°, and Z= 1; (4), monoclinic, space group A2/m, a= 7.709(7), b= 15.490(8), c= 8.172(3)Å, β= 105.51(5)°, and Z= 2. The structures have been solved by the heavy-atom method and refined to R values of 0.041 (1), 0.029 (2), 0.032 (3), and 0.065 (4) using, respectively, 1 787, 1 892, 1 600, and 594 diffractometer-measured intensities. The X-ray crystallographic results together with literature data have been utilised to examine a general relationship between the donor ability of the axial ligands and the length of the Cu–C ubond and draw some conclusions regarding the nature of this bond.