The hydrogenation of ethyl acetate to ethanol catalyzed by SNS pincer ruthenium complexes was computationally investigated by using DFT. Different from a previously proposed mechanism with fac-[(SNS)Ru(PPh₃)(H)₂] (5′) as the catalyst, an unexpected direct hydride transfer mechanism with a mer-SNS ruthenium complex as the catalyst, and two cascade catalytic cycles for hydrogenations of ethyl acetate to aldehyde and aldehyde to ethanol, is proposed base on our calculations. The new mechanism features ethanol-assisted proton transfer for H₂ cleavage, direct hydride transfer from ruthenium to the carbonyl carbon, and C−OEt bond cleavage. Calculation results indicate that the rate-determining step in the whole catalytic reaction is the transfer of a hydride from ruthenium to the carbonyl carbon of ethyl acetate, with a total free energy barrier of only 26.9 kcal mol⁻¹, which is consistent with experimental observations and significantly lower than the relative free energy of an intermediate in a previously postulated mechanism with 5′ as the catalyst.

Inspired by the active-site structure of the [NiFe] hydrogenase, we have computationally designed the iron complex [P^tBu₂N^tBu₂)Fe(CN)₂CO] by using an experimentally ready-made diphosphine ligand with pendant amines for the hydrogenation of CO₂ to methanol. Density functional theory calculations indicate that the rate-determining step in the whole catalytic reaction is the direct hydride transfer from the Fe center to the carbon atom in the formic acid with a total free energy barrier of 28.4 kcal mol⁻¹ in aqueous solution. Such a barrier indicates that the designed iron complex is a promising low-cost catalyst for the formation of methanol from CO₂ and H₂ under mild conditions. The key role of the diphosphine ligand with pendent amine groups in the reaction is the assistance of the cleavage of H₂ by forming a Fe−H^δ−⋅⋅⋅H^δ+−N dihydrogen bond in a fashion of frustrated Lewis pairs.