Abstract IR spectral data reveal that the 18-electron species (C 5H 5) 2Cr(CO) is reversibly transformed, at elevated CO pressures into a dicarbonyl complex, (C 5H 5) 2Cr(CO) 2. Close coincidence of the v(CO) absorption bands of this dicarbonyl with those of (η 3-C 5H 5)(η 5-C 5H 5)W(CO) 2 and of η 3-C 5H 7(η 5-C 5H 5)Cr(CO) 2 indicates that it contains an η 3-bound ring ligand. Equlibrium and kinetic data have been obtained for this CO-induced ring-slippage reaction and for an analogous reaction involving the tetramethylethanediyl-bridged ansa-chromocene carbonyl complex (CH 3) 4C 2-(C 5H 4) 2Cr(CO). The CO-induced transformation of this complex to (CH 3) 4C 2(η 3-C 5H 4)(η 5-C 5H 4)Cr(CO) 2 is greatly accelerated by small amounts of oxidants, i.e. by the formation of the cationic carbonyl complex (CH 3) 4C 2(C 5H 4) 2Cr(CO) +, which appears to facilitate the CO-induced ring slippage by electron-transfer catalysis.