A di(mu-phenoxo)(CoCuI)-Cu-II Complex, [CoCu(L)]ClO4. 0.5DMF, has been prepared where (L)(2-) is a macrocyclic dinucleating compartmental ligand derived from the 2:1:1 condensation of 2,6-diformyl-4-methylphenol, ethylenediamine, and 3-thia-1,5-pentanediamine, having a salen-like N2O2 metal-binding site (salen = N,N'-ethylenedisalicylideneaminate ) and an N2O2S site sharing the phenolic oxygens. It crystallizes in the monoclinic space group P2(1)/n, a = 7.087(1) Angstrom, b = 19.593(6) Angstrom, c 19.447(3) Angstrom, beta = 96.29(1)degrees, V = 2684(1) Angstrom(3), and Z = 4. The Co-II resides in the salen-like site and assumes a planar geometry. The Cu-I in the N2O2S site has a planar four-coordinate geometry with two phenolic oxygens and two imine nitrogens; the thioether sulfur on the lateral chain is situated at an axial site of the Cu-I but the Cu-S separation is very large (3.08 Angstrom). In the {CuN2O2} chromophore, the two Cu-O bond distances (2.333(5) and 2.385(5) Angstrom) an significantly long relative to the two Cu-N bond distances (1.984(6) and 1.929(4) Angstrom). Further, the N-Cu-N angle is very large (128.2(2)degrees), whereas the O-Cu-O angle is very small (65.2(2)degrees). Analogous (CuCuI)-Cu-II and (CoPbII)-Pb-II complexes of (L)(2-), [(CuCuI)-Cu-II(L)]-ClO4 and [CoPb(L)](ClO4)(2), have been prepared. The DMF adduct of [(CuCuI)-Cu-II(L)]ClO4, [(CuCuI)-Cu-II(L)]ClO4. 0.5DMF, crystallizes in the monoclinic space group P2(1)/n, a = 6.957(2) Angstrom, b = 27.774(2) Angstrom, c 7.758(3) Angstrom, beta = 94.72(3)degrees, V = 1278.8(6) Angstrom(3), and Z = 4. The dinuclear core resembles that of the (CoCuI)-Cu-II complex and shows distortions about the Cu-I similar to those found for the (CoCuI)-Cu-II complex. The di-DMF adduct of the (CoPbII)-Pb-II complex, [CoPb(L)(DMF)(2)](ClO4)(2), crystallizes in the monoclinic space group P2(1)/n, a = 16.265(2) Angstrom, b = 12.640(2) Angstrom, c = 18.583(3) Angstrom, beta = 90.43(1)degrees, V = 3820.5(8) Angstrom(3), and Z = 4. The Co-II in the N2O2 site has a square-pyramidal geometry together with a DMF oxygen at the apical site. The Pb-II in the N2O2S site has a six-coordinate geometry with the further coordination of a DMF molecule. The reactivity of the (CoCuI)-Cu-II complex toward dioxygen has been studied in comparison with the (CuCuI)-Cu-II and (CoPbII)-Pb-II complexes. The (CoCuI)-Cu-II complex in DMF is very sensitive to dioxygen and is oxidized at -50 degrees C. The (CuCuI)-Cu-II complex is inert to dioxygen. The (CoPbII)-Pb-II complex is oxygenated at 0 degrees C to form a peroxo dimer ((PbCoIII)-Co-II-O-O-(CoPbII)-Pb-III). The high sensitivity of the (CoCuI)-Cu-II complex to dioxygen is explained by the irreversible oxidation to a (CoCuII)-Cu-III species through an intramolecular-type peroxo complex (Co-III-O-O-Cu-II).