The crystal structures of the adducts of $CU_2(OCOCH_3)_4$ with methanol (l), 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 $CU_2(OCOCH_3)_4.2H_2O$ 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 disorderedm around the N-N axis. Crystal data : (I), monoclinic, space group $P2_1/n$, a = 8.129(2), b = 7.447(1), c = 13.332(1) \AA, \beta = 92.21$(1)^o$, and Z = 2 ; (2), space group, $P2_1/n$, a = 15.153(2), b = 7.772(1), c = 8.229(1) \AA, \beta = $103.08(1)^o$, and Z = 2; (3), triclinic, space group $P^-_1$, a = 8.002(1), b = 8.140(1), c = 9.394(2) \AA, \alpha = 105.85(2), \beta = 92.31 (2), \gamma = $113.84(1)^o$, and Z = 1; (4), monoclinic, space group A2/m, a = 7.709(7), b = 15.490(8), c = 8.1 72(3) \AA, \beta = $105.51 (5)^o$, and Z = 2. The structures have been solved by the heavy-atom method and refined to R values of 0.041 (l), 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-Cu bond and draw some conclusions regarding the nature of this bond.