The chelate assisted oxidative addition of the H S bond of L-cysteine esters HSCH 2 C * H(COOR)NH 2 (R = Me, Et), to Pt(PPh 3 ) 4 yields the low symmetry cysteine complexes [Pt(H)(SCH 2 C * H(COOR)NH 2 )(PPh 3 )] in one step, with displacement of three moles of triphenylphosphine, only when an excess of the cysteine ester is used. With HSCH 2 CH 2 NH 2 the excess is not necessary: the 1:1 reaction yields [Pt(H)(SCH 2 CH 2 NH 2 )(PPh 3 )]. Bulkier HSCH 2 CH 2 NMe 2 and HSCMe 2 CH(COOMe)NH 2 (penicillamine methyl ester) do not react, even when an excess of ligand is used. 2-Aminothiophenol gives trans-[Pt(H) (SC 6 H 4 -2-NH 2 ) (PPh 3 ) 2 ] and does not form a chelate complex.