Abstract: The invention relates to a bidentate phosphine ligand, the phosphorus atoms of the phosphine being connected via a bridge group, the bridge group of the bidentate phosphine ligand consisting of an ortho-anellated annular system, comprising two aryl groups, which aryl groups are connected by two bridges, the first bridge consisting of an --O-- or an --S-- atom and the second bridge being a group that contains an oxygen, sulphur, nitrogen, silicon or carbon atom or a combination of these atoms, the two phosphorus atoms being connected to the two aryl groups of the bridge group at the ortho-position relative to the --O-- or --S-- atom of the first bridge.The invention also relates to the use of this bidentate phosphium ligand in a catalyst system comprising also a transition metal compound which is used in the following type of reactions: hydroformylation hydrogenation, hydrocyanation, polymerization, isomerisation, carboxylation, cross coupling and metathesis.