The synthesis and catalytic behavior of the osmium(II) complexes [OsCl (η -p-cymene)(PR OH)] [R=Me (2 a), Ph (2 b), OMe (2 c), OPh (2 d)] in nitrile hydration reactions is presented. Among them, the best catalytic results were obtained with the phosphinous acid derivative [OsCl (η -p-cymene)(PMe OH)] (2 a), which selectively provided the desired primary amides in excellent yields and short times at 80 °C, employing directly water as solvent, and without the assistance of any basic additive (TOF values up to 200 h ). The process was successful with aromatic, heteroaromatic, aliphatic, and α,β-unsaturated organonitriles, and showed a high functional group tolerance. Indeed, complex 2 a represents the most active and versatile osmium-based catalyst for the hydration of nitriles reported so far in the literature. In addition, it exhibits a catalytic performance similar to that of its ruthenium analogue [RuCl (η -p-cymene)(PMe OH)] (4). However, when compared to 4, the osmium complex 2 a turned out to be faster in the hydration of less-reactive aliphatic nitriles, whereas the opposite trend was generally observed with aromatic substrates. DFT calculations suggest that these differences in reactivity are mainly related to the ring strain associated with the key intermediate in the catalytic cycle, that is, a five-membered metallacyclic species generated by intramolecular addition of the hydroxyl group of the phosphinous acid ligand to the metal-coordinated nitrile.

The utility of chlorophosphines as auxiliary ligands in metal-catalyzed nitrile hydration reactions has been demonstrated, along with their application in the preparation of β-ketoamides.

Three different series of half‐sandwich Os(II) complexes, namely [OsCl2(η6‐p‐cymene){PPh2(NR2)}] [R=Me (5a), Et (5b)], [OsCl2(η6‐p‐cymene){PPh(NR2)2}] [R=Me (6a), Et (6b)] and [OsCl2(η6‐p‐cymene){P(NR2)3}] [R=Me (7a), Et (7b)], have been synthesized. These species proved to be catalytically active for the selective hydration of organonitriles, with compound [OsCl2(η6‐p‐cymene){PPh2(NMe2)}] (5a) being the most effective. In addition, the catalytic activity of 5a was found to be superior to those shown by related Ru(II), Ru(IV), Rh(I) and Pt(II) species containing the amino‐phosphane PPh2(NMe2), i. e. compounds [RuCl2(η6‐p‐cymene){PPh2(NMe2)}] (10), [RuCl2(η3:η3‐C10H16){PPh2(NMe2)}] (12), [RhCl(COD){PPh2(NMe2)}] (13) and cis‐[PtCl2{PPh2(NMe2)}2] (15). Details on the synthesis and characterization of complexes 10, 12 and 15 are also included, along with a discussion on the role played by the ligand PPh2(NMe2) during the catalytic reactions. In this sense, the amino‐phosphane does not act as H‐bond acceptor for the water molecule, generating instead in the aqueous reaction medium the cooperative phosphinous acid ligand PPh2(OH).