The five-co-ordinate complex [RuCI,(PPh,){(S)-biphemp}] [(S)-biphemp = (S)-2,2'-bis(diphenyIphosphino) -6.6'-dimethylbiphenyl] reacts with pentane-2.4-dione (Hacac) in the presence of NEt, to give [RuCl(acac)(PPh,){(S)-biphemp}], which has been isolated in the solid state. The reaction of the chlorohydride [RuHCI( PPh,){(S) -biphemp}] with Hacac gives [ RuH (acac) (PPh,){(S) -biphemp}]and eventually [Ru (acac),{ (S) -biphemp}], which were identified spectroscopically. The relevance of the formation of these new species to the enantioselective hydrogenation of 2,4-diketones to the corresponding diols has been studied by testing [RuCI,( PPh,){(S)-biphemp}] and some of its derivatives as catalyst precursors, also in connection with the use of modifiers such as anhydrous HCI, PPh,, chloride ions, NEt, and HBF,. The hydrogenation reaction has also been followed under ambient conditions by means of 31P and 'H NMR spectroscopy. The activity and selectivity data confirm that for optimum efficiency two chloride ligands must be present in the catalyst precursor. These data are discussed in view of the assessment of a possible reaction pathway for the catalytic reaction.

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