Reactions of triphenylphosphane-substituted ethoxycarbonylcarbene-bridged dicobalt carbonyl complexes with carbon monoxide or CO: 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.