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 C_carbene-C_carbonyl bond on the energy of the rotamers was also examined.