Coordination of α,β-unsaturated aldehydes on d6 Ru and Rh complexes: A DFT study

The coordination modes of various α,β-unsaturated aldehydes on d6 ML4 and ML5 complexes of rhodium and ruthenium have been compared by means of density functional theory (DFT) calculations. The studied aldehydes were acrolein, crotonaldehyde, prenal and cinnamaldehyde and the metallic fragments RuH2(PH3)2, RuH2(PH3)3 and RhH2Cl(PH3)2. On the d6 ML4 Ru fragment, the best coordination geometry is η4, where both double bonds are involved. On the d6 ML5 Ru fragment, the η2CC geometry is preferred to the η1O and to the η2CO ones. Finally on the d6 ML5 Rh fragment, the η2CO mode is not stable and only η1O and η2CC exist, the former being favored. In conclusion, the η2CO coordination geometry is never favored even if the energy gap between η2CC and η2CO is reduced by the use of bulky ligands (PPh3) or alkyl substituents on the Cdouble bond; length as m-dashC bond. The Ru and Rh complexes behave differently: in the case of Ru, η2CC and η2CO can be in competition and in the case of Rh, the best form is η1O. This different behavior can explain the results obtained in the hydrogenation reaction of α,β-unsaturated aldehydes.