The alpha-ketocarboxylatocopper(II) complex [{Cu(L1)}{O2CC(O)CH(CH3)(2)}] can be spontaneously converted into the binuclear oxalatocopper(II) complex [{Cu(L1)}(2)(mu-C2O4)] upon exposure to O-2/CO2 gas. C-13-labeling experiments revealed that oxalate ions partially incorporated (CO2)-C-13 molecules. Furthermore, the bicarbonatocopper(I) complex (NEt4) [Cu(L1){O2C-(OH)}] in an Ar atmosphere and the alpha-ketocarboxylatocopper(1) complex Na[Cu(L1){O2CC-(O)CH(CH3)(2)}] in an 02 atmosphere were also transformed spontaneously into the oxalato complex [{Cu(L1)}2(mu-C2O4)].