The promoting effect of alcohols in the catalytic hydrogenation of CO₂ to formic acid with the solvento complex [TpRu(PPh₃)₂(H₂O)]BF₄ [Tp＝hydrotris(pyrazolyl) borate] is observed. High-pressure NMR monitoring of the catalytic reactions shows that the observable intermediate is a formate complex TpRu(PPh₃)₂(η¹-OCHO)•HOCH₃, which is formed from CO₂ insertion into Ru—H bond of TpRu(PPh₃)₂H and stabilized by hydrogen-bonding interaction between the for-mato ligand and a methanol molecule. The metal formato species comes into equilibrium with another metal formate TpRu(PPh₃)(CH₃OH)(η¹-OCHO); the second metal formato species contains a coordinated methanol, which is intramolecularly hydrogen-bonded with the for-mato ligand. In view of the stability of these two formates under catalytic conditions, it is very likely that they are not within the major catalytic cycle of the reaction. The productive catalytic cycle of the reactions conducted in a variety of alcohols is proposed. The key species in the cycle is the transient alcohol hydride intermediate, TpRu(PPh₃)(ROH)H. It is proposed that TpRu(PPh₃)(ROH)H is able to transfer a hydride and a proton simultaneously to an approaching CO₂ molecule to pro-duce formic acid, itself being converted to a transient alkoxo species, which then associates a H₂ molecule to regenerate TpRu(PPh₃)(ROH)H via σ-metathesis between the alkoxo and η¹-H₂ ligands.

Key words: ruthenium complex, alcohol effect, in situ high-pressure NMR, catalytic mechanism