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

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The coordination modes of various α,β-unsaturated aldehydes on d 6 ML 4 and ML 5 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 RuH 2(PH 3) 2, RuH 2(PH 3) 3 and RhH 2Cl(PH 3) 2. On the d 6 ML 4 Ru fragment, the best coordination geometry is η 4, where both double bonds are involved. On the d 6 ML 5 Ru fragment, the η 2CC geometry is preferred to the η 1O and to the η 2CO ones. Finally on the d 6 ML 5 Rh fragment, the η 2CO mode is not stable and only η 1O and η 2CC exist, the former being favored. In conclusion, the η 2CO coordination geometry is never favored even if the energy gap between η 2CC and η 2CO is reduced by the use of bulky ligands (PPh 3) or alkyl substituents on the C C bond. The Ru and Rh complexes behave differently: in the case of Ru, η 2CC and η 2CO can be in competition and in the case of Rh, the best form is η 1O. This different behavior can explain the results obtained in the hydrogenation reaction of α,β-unsaturated aldehydes.