Reactions of triphenylphosphane-substituted ethoxycarbonylcarbene-bridged dicobalt carbonyl complexes with carbon monoxide or 13CO: An experimental and theoretical study

In CH2Cl2 solution and under a carbon monoxide atmosphere the cobalt complexes [μ2-{ethoxycarbonyl(methylene)}-μ2-(carbonyl)-bis(triphenylphosphanedicarbonyl-cobalt) (Co–Co)] (4) and [μ2-{ethoxycarbonyl(methylene)}-μ2-(carbonyl)-(tricarbonyl-cobalt)-(triphenylphosphanedicarbonyl-cobalt) (Co–Co)] (3) are in equilibrium. The equilibrium constant K = [3][PPh3]/[4][CO] at 10 °C is 1.03 ± 0.11. The bridging and terminal CO ligands in complex 3 or 4 exchange with external 13CO simultaneously. In accord with that variable-temperature 13C NMR spectra reveal fluxional behavior for both complexes. The overall rate constant of 13CO-exchange for 3 at 10 °C is 17 × 10−3 s−1 and for 4 at 10 °C is 26 × 103 s−1. In the case of complex 4 the concentration of PPh3 has practically no influence on the rate of the 13CO-exchange reaction and on the rate of the reaction with CO. The coupling of the μ2-ethoxycarbonylcarbene ligand and one of the coordinated carbon monoxide is at least one order of magnitude slower than the 13CO-exchange reactions, and is faster in complex 4 than in complex 3. The partial pressure of carbon monoxide has practically no effect on the coupling reaction.