Compounds of formula I ##STR1## wherein R.sub.1 is C.sub.1 -C.sub.8 alkyl, phenyl or phenyl which is substituted by 1 to 3 C.sub.1 -C.sub.4 alkyl or C.sub.1 -C.sub.4 alkoxy groups; R.sub.2 and R.sub.3 are identical and are C.sub.1 -C.sub.12 alkyl, C.sub.5 -C.sub.12 cycloalkyl or C.sub.1 -C.sub.4 alkyl- or C.sub.1 -C.sub.4 alkoxy-substituted C.sub.5 -C.sub.12 cycloalkyl, or phenyl which is substituted by one to three identical or different members selected from the group consisting of C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy, --SiR.sub.4 R.sub.5 R.sub.6, halogen, --SO.sub.3 M, --CO.sub.2 M, --PO.sub.3 M,--NR.sub.7 R.sub.8 and --[.sup..sym. NR.sub.7 R.sub.8 R.sub.9 ]X.sup..crclbar. ; or R.sub.2 and R.sub.3 are different and are C.sub.1 -C.sub.12 alkyl, C.sub.5 -C.sub.12 cycloalkyl, C.sub.1 -C.sub.4 alkyl- or C.sub.1 -C.sub.4 alkoxy-substituted C.sub.5 -C.sub.12 cycloalkyl, phenyl or phenyl which is substituted by one to three identical or different members selected from the group consisting of C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy, --SiR.sub.4 R.sub.5 R.sub.6, halogen, --SO.sub.3 M, --CO.sub.2 M, --PO.sub.3 M.sub.2, --NR.sub.7 R.sub.8 and --[.sup..sym.NR.sub.7 R.sub.8 R.sub.9 ]X.sup..crclbar. ; or the group --PR.sub.2 R.sub.3 is a radical of formula II ##STR2## and R.sub.4, R.sub.5 and R.sub.6 are each independently of one another C.sub.1 -C.sub.12 alkyl or phenyl, R.sub.7 and R.sub.8 are H, C.sub.1 -C.sub.2 alkyl, phenyl or R.sub.7 and R.sub.8, taken together, are tetramethylene, pentamethylene or 3-oxa-1,5-pentylene, R.sub.9 is H or C.sub.1 -C.sub.4 alkyl, M is H or an alkali metal, X.sup..crclbar. is the anion of a monobasic acid, and * is a stereogenic carbon atom, in the form of their racemates and diastereoisomers or mixtures of diastereoisomers. Rhodium and iridium complexes with these ligands are suitable for use as homogeneous enantioselective catalysts for the hydrogenation of prochiral compounds containing carbon double bonds or carbon/hetero atom double bonds.