Phosphorous acid P,P′-[5,5′,6,6′-tetramethyl-3,3′-bis(l- methylethyl)[1,1′-biphenyl]-2,2′-diyl] P,P,P′,P'-tetrakis(2,4-dimethylphenyl) ester in hydroformylation

A symmetric bidentate metal ligand complex mixture comprising a compound represented by formula (2):

• and a metal atom selected from the group consisting of Rh, Ru, Co and Ir, wherein the symmetric bidentate metal ligand complex mixture has a chlorine value according to Wickhold below 10,000 ppm.