Internal carbon monoxide exchange and CO dissociation in cobalt carbonyl carbene complexes. A density functional study

Structures, intramolecular CO-exchanges, and CO-dissociation of ethoxycarbonylcarbene-bridged dicobalt carbonyl complexes [μ2-{ethoxycarbonyl(methylene)}-μ2-(carbonyl)- bis(tricarbonyl-cobalt) (Co–Co)] Co2(CO)7(CHCO2Et) (1) and [di-μ2-{ethoxycarbonyl(methylene)}-bis(tricarbonyl-cobalt) (Co–Co)] Co2(CO)6(CHCO2Et)2 (2) were investigated by utilizing the density functional theory at the B3LYP/6-31G(d) level. In the lowest energy isomer of 1 the equatorial carbonyl group cis to the bridging ethoxycarbonylcarbene is prone to dissociate resulting in a coordinative unsaturated Co2(CO)6(CHCO2Et) complex stabilized by an intramolecular cobalt–oxygen orbital interaction. Several mechanisms describing the fluxional behavior of 1 and 2 were found. It was found that the internal transformation designated as ‘tripodal rotation’ is responsible for the temperature-dependent broadening of the 13C NMR signals for compound 2. For 1 the tripodal rotation needs to be taken into account as well, however an even faster internal CO-exchange allows for the carbonyls to switch between the terminal and bridging positions. In the coordinative unsaturated complexes Co2(CO)6(CHCO2Et) and Co2(CO)5(CHCO2Et)2 the CO ligands show also many variations of internal rearrangements. In complex 1 the effect of the rotation of the Ccarbene–Ccarbonyl bond on the energy of the rotamers was also examined.