ansa-metallocene derivatives : XV. Reversible coordination of a second CO ligand to (C₅H₅)₂Cr(CO) and its tetramethylethanediyl-bridged analogue. Evidence for an electron-transfer catalysed ring-slippage reaction. IR studies at elevated gas pressure. II

Abstract

IR spectral data reveal that the 18-electron species (C5H5)2Cr(CO) is reversibly transformed, at elevated CO pressures into a dicarbonyl complex, (C5H5)2Cr(CO)2. Close coincidence of the v(CO) absorption bands of this dicarbonyl with those of (η3-C5H5)(η5-C5H5)W(CO)2 and of η3-C5H7(η5-C5H5)Cr(CO)2 indicates that it contains an η3-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 (CH3)4C2-(C5H4)2Cr(CO). The CO-induced transformation of this complex to (CH3)4C2(η3-C5H4)(η5-C5H4)Cr(CO)2 is greatly accelerated by small amounts of oxidants, i.e. by the formation of the cationic carbonyl complex (CH3)4C2(C5H4)2Cr(CO)+, which appears to facilitate the CO-induced ring slippage by electron-transfer catalysis.