Abstract: A catalyst of ruthenium (II) which includes bidentate phosphine ligands. The catalyst is obtainable by a process which includes the steps of treating equimolar amounts of an appropriate Ru(II) complex and a bidentate diphosphine ligand with an acid of formula HX, wherein X is a non-coordinating anion. The acid is used in a ratio which does not exceed 2 molar equivalents per mole of Ru(II) complex and the treatment is carried out in a non-coordinating or weakly coordinating solvent and under an inert atmosphere. This catalyst is useful for the preparation of the preferred isomer of HEDIONE, having the (+)-(1R)-cis-configuration.