Formyl complexes thermodynamic stability

Now that we have an eflScient route to metal formyl complexes, we can study the thermodynamic stability of these complexes. We have found that metal formyls are much less thermodynamically stable than the corresponding metal hydrides. Thus, metal formyl species have never been observed in the reactions of metal hydrides with CO because of the thermodynamic instability of the formyl complexes and not because of their kinetic instability. 

The greatly different thermodynamic stabilities of metal acyl and metal formyl compounds is probably attributable to the greater strength of the M-H bond (estimated 50-60 kcal mol ) compared with the M-C bond (estimated 30-40 kcal mol" ). We are now attempting to measure the heat of reaction for the conversion of a metal-formyl compound to a metal hydride. This will allow a much better estimate of the energy difference between a metal formyl complex and a metal hydride since the energy difference was too large to measure by equilibration of the species. 

Similarly, CO insertion (or migratory insertion) is the fundamental transformation of the CO ligand. It is a reversible proeess and an equilibrium will exist between the Tq -alkyl and nq -acyl complexes (Scheme 1.11). This insertion is not observed for the formation of formyl ligands (R = H), owing to the low thermodynamic stability of the formyl group. 

Representative work A Dedieu and F. Ingold, Angew. Chem., Int. Ed. Engl., 28, 1694 (1989). Monohapto Versus Dihapto Coordination of Carbon Dioxide in Bis(ammine) Nickel(O) Complexes. A CAS-SCF Investigation. C. Bo and A. Dedieu, Inorg. Chem., 28, 304 (1989). CO2 Interaction with HCr(CO)J Theoretical Study of the Thermodynamic Aspects. A. Dedieu and S. Nakamura, J. Organomet. Chem., 260, C63 (1984). lative Thermodynamic Stabilities of Acetyl and Formyl Complexes A Theoretical Determination. 

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