The crystal and mol. structure of the title compds. were detd. at low temps. The accuracy of the structures allows for a discussion about the bond alternation in the six-membered ring. Both complexes show bond localization, whereas the benzocyclobutene-Cr(CO)3 shows localization in the Mills-Nixon effect manner, the benzocyclobutadiene-Fe(CO)3 shows an anti-Mills-Nixon localization.