Photolysis of (HBpz(3))ReO(C2O4) (HBpz(3) = hydrotris(1-pyrazolyl)borate) in hexafluoroacetone/acetonitrile solution in the presence of dioxygen forms I, on the basis of an X-ray crystal structure and spectroscopic data. The unusual and complex metallacycle is constructed from two molecules of hexafluoroacetone, one molecule of acetonitrile, and half of the original oxalate ligand. The formation of the metallacycle and the results of an isotope-labeling study are rationalized by a mechanism involving a cascade of electrocyclic reactions.